Croatia, officially the Republic of Croatia, is a sovereign state between Central Europe, Southeast Europe, and the Mediterranean. Its capital city is Zagreb, which forms one of the country's primary subdivisions, along with its twenty counties. Croatia covers 56,594 square kilometres (21,851 square miles) and has diverse, mostly continental and Mediterranean climates. Croatia's Adriatic Sea coast contains more than a thousand islands. The country's population is 4.28 million, most of whom are Croats, with the most common religious denomination being Roman Catholicism. The Croats arrived in the area of present-day Croatia during the early part of the 7th century AD. They organised the state into two duchies by the 9th century.