US President Barack Obama, center, poses for a group photograph Sunday, November 20, at the summit of the Asia-Pacific Economic Cooperation. The summit was held in Lima, Peru. Joining Obama in this photo, from left, are Thai Deputy Prime Minister Prajin Juntong, Japanese Prime Minister Shinzo Abe, South Korean Prime Minister Hwang Kyo-ahn and Vietnamese President Tran Dai Quang. Obama is on his last international trip as president. He made earlier stops in Greece and Germany. Mikhail Metzel/TASS/Getty Images

Obama addresses a town-hall meeting in Lima for the Young Leaders of the Americas Initiative. Guillermo Gutierrez/Bloomberg/Getty Images

Obama takes a question during the town-hall meeting, which took place on Saturday, November 19. BRENDAN SMIALOWSKI/AFP/Getty Images

Cyntia Paytan greets Obama after introducing him at the town-hall meeting. BRENDAN SMIALOWSKI/AFP/Getty Images

Obama and other leaders wait for a meeting November 19 during the summit of the Asia-Pacific Economic Cooperation. BRENDAN SMIALOWSKI/AFP/Getty Images

Obama confers with Peruvian President Pedro Pablo Kuczynski during the summit on November 19. Brendan Smialowski/AFP/Getty Images

Mercedes Araoz, Peru's second vice president, joins Obama on the tarmac after his arrival at an airport near Lima on Friday, November 18. Pablo Martinez Monsivais/AP

On his way to Peru, Obama boards Air Force One during a refueling stop in the Azores on November 18. Pablo Martinez Monsivais/AP

Obama walks across the tarmac during the refueling stop in the Azores. Pablo Martinez Monsivais/AP

Obama meets with, from left, British Prime Minister Theresa May, Spanish Prime Minister Mariano Rajoy, Italian Prime Minister Matteo Renzi, French President François Hollande and German Chancellor Angela Merkel at the German Chancellery in Berlin on November 18. Pablo Martinez Monsivais/AP

Merkel welcomes Obama before a meeting in Berlin on Thursday, November 17. Michael Sohn/AP

Obama walks past the Brandenburg Gate after visiting the US Embassy in Berlin on November 17. Pablo Martinez Monsivais/AP

Obama is greeted by a guard of honor after flying into Berlin on Wednesday, November 16. TOBIAS SCHWARZ/AFP/Getty Images

Obama waves to the crowd after delivering a speech at the Stavros Niarchos Foundation in Athens, Greece, on November 16. Aggelos Barai/NurPhoto via Getty Images

Obama tours the Acropolis in Athens on November 16. Pablo Martinez Monsivais/AP

Obama meets with Greek President Prokopis Pavlopoulos at the presidential mansion in Athens on Tuesday, November 15. Obama called Greece a "reliable ally" in its commitment to NATO, even as the country faces tremendous strain from its debt crisis. NurPhoto/NurPhoto/NurPhoto via Getty Images

Obama and Greek Prime Minister Alexis Tsipras field questions at a joint news conference in Athens on November 15. Pablo Martinez Monsivais/AP

Tsipras welcomes Obama before formal talks in Athens on November 15. ARIS MESSINIS/AFP/Getty Images

Obama reviews a presidential honor guard during an official welcoming ceremony in Athens on November 15. LOUISA GOULIAMAKI/AFP/Getty Images

Obama arrives at Athens International Airport on November 15. Thanassis Stavrakis/AP

Greek presidential guards parade outside the presidential mansion in Athens on November 15. LOUISA GOULIAMAKI/AFP/Getty Images