Busan is divided into 15 major administrative districts and a single county, together housing a population of approximately 3.6 million. The full metropolitan area, including the adjacent cities of Gimhae and Yangsan, has a population of approximately 4.6 million. The most densely built-up areas of the city are situated in a number of narrow valleys between the Nakdong and the Suyeong Rivers, with mountains separating most of the districts. The Nakdong is Korea's longest river and Busan's Haeundae Beach is also the country's largest.