Poland is a country in Central Europe bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north. The total area of Poland is 312,679 square kilometers (120,726 sq mi), making it the 69th largest country in the world and the 9th largest in Europe. Poland has a population of over 38 million people, which makes it the 34th most populous country in the world and the sixth most populous member of the European Union, being its most populous post-communist member. The establishment of a Polish state is often identified with the adoption of Christianity by its ruler Mieszko I in 966, over the territory similar to that of present-day Poland. The Kingdom of Poland was formed in 1025, and in 1569 it cemented a long association with the Grand Duchy of Lithuania by signing the Union of Lublin, forming the Polish–Lithuanian Commonwealth.