US President Barack Obama (R) and British Prime Minister David Cameron pose for a picture with Denmark's Prime Minister Helle Thorning Schmidt (C) next to US First Lady Michelle Obama (R) during the memorial service of South African former president Nelson Mandela at the FNB Stadium (Soccer City) in Johannesburg on December 10, 2013. Mandela, the revered icon of the anti-apartheid struggle in South Africa and one of the towering political figures of the 20th century, died in Johannesburg on December 5 at age 95. AFP PHOTO / ROBERTO SCHMIDT

(2ndraw from 2ndL) Danish Prime Minister Helle Thorning-Schmidt, US President Barack Obama and First Lady Michelle Obama attend the memorial service for late South African President Nelson Mandela at Soccer City Stadium in Johannesburg on December 10, 2013. Mandela, the revered icon of the anti-apartheid struggle in South Africa and one of the towering political figures of the 20th century, died in Johannesburg on December 5 at age 95. Mandela, who was elected South Africa's first black president after spending nearly three decades in prison, had been receiving treatment for a lung infection at his Johannesburg home since September, after three months in hospital in a critical state. AFP PHOTO / ALEXANDER JOE

Danish Prime Minister Helle Thorning-Schmidt (L) talks with US President Barack Obama (R) in the Soccer City Stadium in Johannesburg as they attend the memorial service for late South African President Nelson Mandela on December 10, 2013. Mandela, the revered icon of the anti-apartheid struggle in South Africa and one of the towering political figures of the 20th century, died in Johannesburg on December 5 at age 95. Mandela, who was elected South Africa's first black president after spending nearly three decades in prison, had been receiving treatment for a lung infection at his Johannesburg home since September, after three months in hospital in a critical state. AFP PHOTO / ELMOND-JIYANE

ALTERNATIVE CROP US President Barack Obama (R) and British Prime Minister David Cameron pose for a picture with Denmark's Prime Minister Helle Thorning Schmidt (C) next to US First Lady Michelle Obama (R) during the memorial service of South African former president Nelson Mandela at the FNB Stadium (Soccer City) in Johannesburg on December 10, 2013. Mandela, the revered icon of the anti-apartheid struggle in South Africa and one of the towering political figures of the 20th century, died in Johannesburg on December 5 at age 95. AFP PHOTO / ROBERTO SCHMIDT

US President Barack Obama (R) and British Prime Minister David Cameron pose for a picture with Denmark's Prime Minister Helle Thorning Schmidt (C) during the memorial service of South African former president Nelson Mandela at the FNB Stadium (Soccer City) in Johannesburg on December 10, 2013. Mandela, the revered icon of the anti-apartheid struggle in South Africa and one of the towering political figures of the 20th century, died in Johannesburg on December 5 at age 95. AFP PHOTO / ROBERTO SCHMIDT

US President Barack Obama (R) smiles before posing for a picture with British Prime Minister David Cameron and Denmark's Prime Minister Helle Thorning Schmidt (C) during the memorial service of South African former president Nelson Mandela at the FNB Stadium (Soccer City) in Johannesburg on December 10, 2013. Mandela, the revered icon of the anti-apartheid struggle in South Africa and one of the towering political figures of the 20th century, died in Johannesburg on December 5 at age 95. AFP PHOTO / ROBERTO SCHMIDT

US President Barack Obama (R), British Prime Minister David Cameron look at the picture taken by Denmark's Prime Minister Helle Thorning Schmidt (C) during the memorial service of South African former president Nelson Mandela at the FNB Stadium (Soccer City) in Johannesburg on December 10, 2013. Mandela, the revered icon of the anti-apartheid struggle in South Africa and one of the towering political figures of the 20th century, died in Johannesburg on December 5 at age 95. AFP PHOTO / ROBERTO SCHMIDT

This combo of pictures shows US President Barack Obama (R) and British Prime Minister David Cameron (L) posing for a photo with Denmark's Prime Minister Helle Thorning Schmidt (C) during the memorial service of South African former president Nelson Mandela at the FNB Stadium (Soccer City) in Johannesburg on December 10, 2013. Mandela, the revered icon of the anti-apartheid struggle in South Africa and one of the towering political figures of the 20th century, died in Johannesburg on December 5 at age 95. AFP PHOTO / ROBERTO SCHMIDT

Denmark's Prime Minister Helle Thorning Schmidt (top L) and US President Barack Obama (C) and US First Lady Michelle Obama (top R) for the memorial service of South African former president Nelson Mandela at the FNB Stadium (Soccer City) in Johannesburg on December 10, 2013. Mandela, the revered icon of the anti-apartheid struggle in South Africa and one of the towering political figures of the 20th century, died in Johannesburg on December 5 at age 95. AFP PHOTO / ALEXANDER JOE

This combo of pictures shows US President Barack Obama (R) and British Prime Minister David Cameron (L) posing for a photo with Denmark's Prime Minister Helle Thorning Schmidt (C) during the memorial service of South African former president Nelson Mandela at the FNB Stadium (Soccer City) in Johannesburg on December 10, 2013. Mandela, the revered icon of the anti-apartheid struggle in South Africa and one of the towering political figures of the 20th century, died in Johannesburg on December 5 at age 95. AFP PHOTO / ROBERTO SCHMIDT

President Barack Obama jokes with Danish prime minister, Helle Thorning-Schmidt, left, as first lady Michelle Obama looks on at right during the memorial service for former South African president Nelson Mandela at the FNB Stadium in Soweto, near Johannesburg, South Africa, Tuesday Dec. 10, 2013. (AP Photo/Matt Dunham), APTOPIX

President Barack Obama jokes with Danish prime minister, Helle Thorning-Schmidt, left, as first lady Michelle Obama looks on at right during the memorial service for former South African president Nelson Mandela at the FNB Stadium in Soweto, near Johannesburg, South Africa, Tuesday Dec. 10, 2013. (AP Photo/Matt Dunham)