The United States of America (USA), commonly known as the United States (U.S. or US) or simply America, is a country consisting of 50 states, a federal district, five major self-governing territories, and various possessions. At 3.8 million square miles (9.8 million km2), it is the world's third or fourth-largest country by total area. Most of the country is located in central North America between Canada and Mexico. With an estimated population of over 328 million, the U.S. is the third most populous country in the world (after China and India). The capital is Washington, D.C., and the most populous city is New York City. Paleo-Indians migrated from Siberia to the North American mainland at least 12,000 years ago. The United States is a federal republic and a representative democracy. The United States is the world's largest importer and the second-largest exporter of goods, by value. Although its population is 4% of the world total, it holds 29.4% of the total wealth in the world, the largest share of global wealth concentrated in a single country. It is the foremost military power in the world, making up more than a third of global military spending, and is a leading political, cultural, and scientific force internationally.