Uruguay is a country located in South America bordering the Southern Atlantic Ocean. Neighboring countries include Argentina and Brazil. The geography of Uruguay includes mostly rolling grassland and a dense network of rivers. The government system is a constitutional republic; the chief of state and head of government is the president. Uruguay has a mixed economic system in which there is a variety of private freedom, combined with centralized economic planning and government regulation. Uruguay is a member of the Common Market of the South (Mercosur).