The United States of America is a constitutional federal republic comprising fifty states and a federal district At 3.79 million square miles (9.83 million km²) and with over 300 million people, the United States is the third or fourth largest country by total area, and third largest by land area and by population. The United States is one of the world's most ethnically diverse nations, the product of large-scale immigration from many countries.The U.S. economy is the largest national economy in the world, with a nominal 2006 gross domestic product (GDP) of more than US$13 trillion. Because of the United States' large size and wide range of geographic features, nearly every type of climate is represented. The United States has vast economic, political, and military influence on a global scale, which makes its foreign policy a subject of great interest around the world. Almost all countries have embassies in Washington, D.C., and many host consulates around the country. Likewise, nearly all nations host American diplomatic missions. However, Cuba, Iran, North Korea, Bhutan, Sudan, and the Republic of China (Taiwan) do not have formal diplomatic relations with the United States. The United States has a capitalist mixed economy, which is fueled by abundant natural resources, a well-developed infrastructure, and high productivity. The United States has been a leader in scientific 'research and technological innovation since the late nineteenth century. . During World War II, the U.S.-based Manhattan Project developed nuclear weapons, ushering in the Atomic Age. Americans enjoy high levels of access to technological consumer goods. The United States is a culturally diverse nation, home to a wide variety of ethnic groups, traditions, and values. Since the early twentieth century, the U.S. film industry has largely been based in and around Hollywood, California. American screen actors like John Wayne and Marilyn Monroe have become iconic figures, while producer/entrepreneur Walt Disney was a leader in both animated film and movie merchandising. The major film studios of Hollywood are the primary source of the most commercially successful movies in the world, such as Star Wars c c(1977) and Titanic (1997), and the products of Hollywood today dominate the global film industry. Americans are the heaviest television viewers in the world,and the average time spent in front of the screen continues to rise, hitting five hours a day in 2006. The rhythmic and lyrical styles of African American music have deeply influenced American music at large, distinguishing it from European traditions. Since the late nineteenth century, baseball has been regarded as the national sport; football, basketball, and ice hockey are the country's three other leading professional team sports. Soccer, though not a leading professional sport in the country, is played widely at the youth and amateur levels. Tennis and many outdoor sports are also popular.