Russian President Dmitry Medvedev, right, and Serbian President Boris Tadic exchange documents during their meeting at the Kremlin in Moscow on Wednesday. Russia and Serbia signed a controversial energy cooperation dealon the sale of a 51 percent stake in the Balkan country's oil monopoly NIS to Russian gas giant Gazprom, in exchange for the construction of a strategic Russian gas pipeline through Serbia. The price of the 51 percent stake NIS was 400 million euros ($560 million), according to Serbian officials.