Saint Kitts and Nevis, part of the Lesser Antilles, is a two-island nation located in the Caribbean Sea. Proximate countries include Antigua and Barbuda, Dominica, and Puerto Rico. Most of the islands are mountainous. The government system is a parliamentary democracy and a Commonwealth realm; the chief of state is the queen of the United Kingdom, and the head of government is the prime minister. Saint Kitts and Nevis has a mixed economic system in which there is a variety of private freedom, combined with centralized economic planning and government regulation. Saint Kitts and Nevis is a member of the Caribbean Community (CARICOM) and the Organization of Eastern Caribbean States (OECS).