President of the Cambodian People's Party (CPP) and Prime Minister Hun Sen (L) shakes hands with CPP Honorary President and President of the National Assembly Heng Samrin as they arrive for a ceremony at the party headquarters to mark the 37th anniversary of the toppling of Pol Pot's Khmer Rouge regime, in Phnom Penh, January 7, 2016. Some 1.7 million people are believed to have died in the "Killing Fields" of the ultra-Maoist guerrillas, whose four year reign of terror was brought to an end in 1979 by invading troops from neighbouring Vietnam. REUTERS/Samrang Pring