US President Barack Obama looks at a computer as he tours the Desmond Tutu HIV Foundation Youth Centre and takes part in a health event in Cape Town, June 30, 2013. Pictured at left is Desmond Tutu and at right is young rapper Aviwe Ntongana, who calls himself 'Katmeister'. [Photo/Agencies]

US President Barack Obama writes in a guest book as he tours Robben Island with first lady Michelle Obama, near Cape Town, June 30, 2013. Under apartheid, Nelson Mandela spent several decades as a political prisoner on Robben Island. [Photo/Agencies]