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LONDON, ENGLAND - MAY 12: Prime Minister David Cameron speaks during the final session at the international anti-corruption summit on May 12, 2016 in London, England. Leaders from many of the worlds nations are gathering in London for the summit, which is aimed at stepping up action to tackle the problem of corruption. (Photo by Stefan Rousseau - WPA Pool/Getty Images)

LONDON, ENGLAND - MAY 12: Prime Minister David Cameron speaks during the final session  at the international anti-corruption summit on May 12, 2016 in London, England. Leaders from many of the worlds nations are gathering in London for the summit, which is aimed at stepping up action to tackle the problem of corruption.  (Photo by Stefan Rousseau - WPA Pool/Getty Images)