LONDON, ENGLAND - APRIL 22: US President Barack Obama and First Lady Michelle Obama are greeted by  Prince Harry,  Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge as they arrive for a dinner at Kensington Palace on April 22, 2016 in London, England.  The President and his wife are currently on a brief visit to the UK where they attended lunch with HM Queen Elizabeth II at Windsor Castle and later will have dinner with Prince William and his wife Catherine, Duchess of Cambridge at Kensington Palace. Mr Obama visited 10 Downing Street this afternoon and held a joint press conference with British Prime Minister David Cameron where he stated his case for the UK to remain inside the European Union. (Photo by Chris Radburn - WPA Pool/Getty Images)