President Obama on Three-Country Tour

Stephan Weil, governor of German state of Lower Saxony, right, welcomes U.S. President Barack Obama, center, upon his arrival at the airport in Hannover, northern Germany, April 24, 2016. Obama is on a two-day official visit to Germany.

Guard of honour arrives before the welcome ceremony of U.S. President Barack Obama at Schloss Herrenhausen in Hanover, Germany, April 24, 2016.

Thousands of demonstrators protest against the planned Transatlantic Trade and Investment Partnership, TTIP, and the Comprehensive Economic and Trade Agreement, CETA, ahead of the visit of U.S. President Barack Obama, in Hannover, Germany, April 23, 2016.

President Barack Obama, 2nd left, greets actors on stage after watching them perform Shakespeare's Hamlet at the Globe Theatre in London, April 23, 2016.

Students raise their hands to ask a question as U.S. President Barack Obama holds a town hall at the Royal Agricultural Halls in London, April 23, 2016.

President Barack Obama with First Lady Michelle Obama meets Prince George as the Duke and Duchess of Cambridge watch at Kensington Palace in London, April 22, 2016.

U.S. President Barack Obama and British Prime Minister David Cameron hold a news conference following their meeting at 10 Downing Street in London, April 22, 2016.

President Barack Obama and his wife first lady Michelle Obama are greeted by Britain's Queen Elizabeth II and Prince Phillip after landing by helicopter at Windsor Castle for a private lunch in Windsor, April, 22, 2016

President Barack Obama, seated upper right, speaks with Abu Dhabi’s Crown Prince Sheikh Mohamed bin Zayed Al Nahyan during a Gulf Cooperation Council session during the Gulf Cooperation Council Summit in Riyadh, Saudi Arabia, April 21, 2016.

U.S. President Barack Obama walks with Saudi King Salman at Erga Palace upon his arrival for a summit meeting in Riyadh, Saudi Arabia, April 20, 2016.