United States President Barack Obama, front, and Dmitry Medvedev, Russia's president, pose for a group photo with the heads of delegations attending the Nuclear Security Summit at the Washington Convention Center in Washington, D.C., U.S., on Tuesday, April 13, 2010. Ukraine's agreement to relinquish its entire stockpile of highly enriched uranium gave Obama the first concrete result for a summit he convened on securing the world's atomic material. .Credit: Andrew Harrer / Pool via CNP
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