Task: Question Understanding
Input (English): Passage: Germany lost a quarter of its pre-war  territory. Among the eastern territories, Silesia, Neumark and most of Pomerania were taken over by Poland, East Prussia was divided between Poland and the USSR, followed by the expulsion of the 9 million Germans from these provinces, as well as the expulsion of 3 million Germans from the Sudetenland in Czechoslovakia to Germany. By the 1950s, every fifth West German was a refugee from the east. The Soviet Union also took over the Polish provinces east of the Curzon line, from which 2 million Poles were expelled; north-east Romania, parts of eastern Finland, and the three Baltic states were also incorporated into the USSR. In an effort to maintain world peace, the Allies formed the United Nations, which officially came into existence on 24 October 1945, and adopted the Universal Declaration of Human Rights in 1948, as a common standard for all member nations. The great powers that were the victors of the war—France, China, Britain, the Soviet Union and the United States—became the permanent members of the UN's Security Council. The five permanent members remain so to the present, although there have been two seat changes, between the Republic of China and the People's Republic of China in 1971, and between the Soviet Union and its successor state, the Russian Federation, following the dissolution of the Soviet Union in 1991. The alliance between the Western Allies and the Soviet Union had begun to deteriorate even before the war was over.
Question: How many years passed between seat changes?
Output (English): number