Former U.S. President Barack Obama is embraced by Nelson Mandela's widow Graca Machel after he to delivered the 16th Nelson Mandela annual lecture in Johannesburg, South Africa July 17, 2018.

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.

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.

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.

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

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.

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.

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.

Former U.S. President Barack Obama chats to South Africa's businnes man 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.

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.

Nelson Mandela's widow Graca Machel addresses crowds gathered to hear former U.S. President Barack Obama delivering the 16th Nelson Mandela annual lecture in Johannesburg, South Africa July 17, 2018.

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.