ITV News' Libby Wiener reports from Sao Paulo in Brazil where Prime Minister David Cameron is on a two-day visit.
The Conservative, who appeared on America's Late Show with David Letterman on Wednesday night, was quizzed by the veteran host over his knowledge of British history.
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The anti-war President is now in combat in two Middle Eastern countries.
Prince Harry received a great big hug from four-year-old Carson Hartley.