Greek Prime Minister Alexis Tsipras, right, arrives for a formal dinner at the Eastern Partnership summit in Riga, on Thursday, May 21, 2015. EU leaders on Thursday will seek new ways to bolster ties with six post-communist nations in Eastern Europe, a year and a half after a previous summit of the Eastern Partnership ended with a fateful standoff over Ukraine. (AP Photo/Mindaugas Kulbis)
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