South African President Cyril Ramaphosa talks to former U.S. President Barack Obama at the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko TPX IMAGES OF THE DAY 

President of South Africa gestures as he arrives for the 16th Nelson Mandela annual lecture, delivered by former U.S. President Barack Obama , marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

President of South Africa gestures as he arrives for the 16th Nelson Mandela annual lecture, delivered by former U.S. President Barack Obama, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Former U.S. President Barack Obama delivers the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Former U.S. President Barack Obama delivers the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Former U.S. President Barack Obama delivers the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko TPX IMAGES OF THE DAY 

South African President Cyril Ramaphosa talks to former U.S. President Barack Obama at the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko TPX IMAGES OF THE DAY 

Guests attend the 16th Nelson Mandela annual lecture delivered by former U.S. President Barack Obama, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Guests attend the 16th Nelson Mandela annual lecture delivered by former U.S. President Barack Obama, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Former U.S. President Barack Obama delivers the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Former U.S. President Barack Obama delivers the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

South African President Cyril Ramaphosa talks to former U.S. President Barack Obama at the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Former U.S. President Barack Obama waves after he delivered the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Former U.S. President Barack Obama chats to South Africa's businessman Patrice Motsepe after Obama delivered the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko/File Photo 

Crowds listen as former U.S. President Barack Obama delivers the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa, July 17, 2018. REUTERS/Siphiwe Sibeko 

Former U.S. President Barack Obama delivers the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa, July 17, 2018. REUTERS/Siphiwe Sibeko 

Former U.S. President Barack Obama delivers the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa, July 17, 2018. REUTERS/Siphiwe Sibeko 

Former U.S. President Barack Obama delivers the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa, July 17, 2018. REUTERS/Siphiwe Sibeko 

Former U.S. President Barack Obama delivers the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa, July 17, 2018. REUTERS/Siphiwe Sibeko 

Crowds listen as former U.S. President Barack Obama delivers the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa, July 17, 2018. REUTERS/Siphiwe Sibeko 

Former U.S. President Barack Obama delivers the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Former U.S. President Barack Obama delivers the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

South African President Cyril Ramaphosa talks to former U.S. President Barack Obama at the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Accompanied by South African President Cyril Ramaphosa, former U.S. President Barack Obama arrives to deliver the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Accompanied by South African President Cyril Ramaphosa, former U.S. President Barack Obama arrives to deliver the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Former U.S. President Barack Obama arrives to deliver the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Human rights lawyer George Bizos arrives to hear Former U.S. President Barack Obama delivering the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Zulu King Goodwill Zwelithini arrives to hear Former U.S. President Barack Obama delivering the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

A man holds a program as crowds gather to hear former U.S. President Barack Obama delivering the 16th Nelson Mandela annual lecture, marking the centenary of the anti-apartheid leader's birth, in Johannesburg, South Africa July 17, 2018. REUTERS/Siphiwe Sibeko 

Protesters clash with riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis TPX IMAGES OF THE DAY

Protesters clash with riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

A petrol bomb explodes next to riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

A petrol bomb explodes next to riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

Protesters clash with riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

Riot police stands on a street amid teargas during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

Protesters clash with riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

Protesters run away from teargas as they clash with riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

Protesters run away from teargas as they clash with riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

Protesters clash with riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

Protesters clash with riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis TPX IMAGES OF THE DAY 

Protesters are confronted by riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

Protesters are confronted by riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

Protesters clash with riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

Protesters clash with riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

Protesters clash with riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis TPX IMAGES OF THE DAY 

Protesters clash with riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

Protesters clash with riot police during a demonstration against the visit of U.S President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

Riot police block a street during a demonstration against the visit of U.S. President Barack Obama, in Athens, Greece, November 15, 2016. REUTERS/Alkis Konstantinidis 

Three anti-abortion protestors are seen outside U.S. Supreme Court building is seen in Washington, March 16, 2016. President Barack Obama will nominate Merrick Garland today to fill the position of deceased Court Justice Antonin Scalia. REUTERS/Jim Bourg

US Secretary of State John Kerry, Italian Foreign Affairs Minister Paolo Gentiloni (R) and US President Barack Obama's special envoy for the international anti-Isil coalition, Brett McGurk (L) attend a ministerial meeting regarding Islamic State in Rome, Italy February 2, 2016. Reuters/Nicholas Kamm/Pool 

A combination photo shows U.S. President Barack Obama delivering State of the Union Address every year during his two terms in office, clockwise from top left, his first State of the Union address in 2009 through January 12, 2016 (bottom L) on Capitol Hill in Washington in these file photos. Obama, delivering his last annual State of the Union speech to Congress as president on Tuesday, called for leaders to "fix" U.S. politics and criticized candidates such as Trump for using anti-Muslim rhetoric that betrayed American values. REUTERS/Files 

U.S. President Barack Obama speaks about counter-terrorism and the United States fight against Islamic State during an address to the nation from the Oval Office of the White House in Washington, December 6, 2015. REUTERS/Saul Loeb/Pool 

U.S. President Barack Obama speaks about counter-terrorism and the United States fight against Islamic State during an address to the nation from the Oval Office of the White House in Washington, December 6, 2015. REUTERS/Saul Loeb/Pool 

U.S. President Barack Obama speaks about counter-terrorism and the United States fight against Islamic State during an address to the nation from the Oval Office of the White House in Washington, December 6, 2015. REUTERS/Saul Loeb/Pool 

U.S. President Barack Obama speaks about counter-terrorism and the United States fight against Islamic State during an address to the nation from the Oval Office of the White House in Washington, December 6, 2015. REUTERS/Saul Loeb/Pool 

U.S. President Barack Obama speaks about counter-terrorism and the United States fight against Islamic State during an address to the nation from the Oval Office of the White House in Washington, December 6, 2015. REUTERS/Saul Loeb/Pool 

U.S. President Barack Obama speaks about counter-terrorism and the United States fight against Islamic State during an address to the nation from the Oval Office of the White House in Washington, December 6, 2015. REUTERS/Yuri Gripas 

U.S. President Barack Obama speaks about counter-terrorism and the United States fight against Islamic State during an address to the nation from the Oval Office of the White House in Washington, December 6, 2015. REUTERS/Saul Loeb/Pool 

U.S. President Barack Obama speaks about counter-terrorism and the United States fight against Islamic State during an address to the nation from the Oval Office of the White House in Washington, December 6, 2015. REUTERS/Yuri Gripas TPX IMAGES OF THE DAY

U.S. President Barack Obama speaks about counter-terrorism and the United States fight against Islamic State during an address to the nation from the Oval Office of the White House in Washington, December 6, 2015. REUTERS/Yuri Gripas