The United States is a country located in North America bordering the Atlantic Ocean and Pacific Ocean. Neighboring countries are Canada and Mexico. The geography of the United States is varied with mountains in the west, a broad central plain, and low mountains in the east. The government system is a constitution-based federal republic with a strong democratic tradition; the chief of state and head of government is the president. The United States has an advanced mixed economy in which there is a variety of private freedom, combined with centralized economic planning and government regulation. United States is a member of the Asia-Pacific Economic Cooperation (APEC) and the North American Free Trade Agreement (NAFTA).