Rwanda, officially the Republic of Rwanda, is a landlocked country in the Great Rift Valley, where the African Great Lakes region and East Africa converge. One of the smallest countries on the African mainland, its capital city is Kigali. Located a few degrees south of the Equator, Rwanda is bordered by Uganda, Tanzania, Burundi, and the Democratic Republic of the Congo. It is highly elevated, giving it the soubriquet "land of thousand hills", with its geography dominated by mountains in the west and savanna to the east, with numerous lakes throughout the country. The climate is temperate to subtropical, with two rainy seasons and two dry seasons each year. Rwanda has a population of over 12.6 million living on 26,338 km (10,169 sq mi) of land, and is the most densely populated mainland African country. The population is young and predominantly rural. Rwandans are drawn from just one cultural and linguistic group, the Banyarwanda. However, within this group there are three subgroups: the Hutu, Tutsi and Twa. The Twa are a forest-dwelling pygmy people and are often considered descendants of Rwanda's earliest inhabitants. Scholars disagree on the origins of and differences between the Hutu and Tutsi; some believe differences are derived from former social castes within a single people, while others believe the Hutu and Tutsi arrived in the country separately, and from different locations. Christianity is the largest religion in the country; the principal language is Kinyarwanda, spoken by most Rwandans, with English and French serving as additional official languages. The sovereign state of Rwanda has a presidential system of government. The president is Paul Kagame of the Rwandan Patriotic Front (RPF), who has served continuously since 2000. Today, Rwanda has low levels of corruption compared with neighbouring countries, although human rights organisations report suppression of opposition groups, intimidation and restrictions on freedom of speech.