The area of modern-day Saudi Arabia formerly consisted of four distinct regions: Hejaz, Najd and parts of Eastern Arabia (Al-Ahsa) and Southern Arabia The Kingdom of Saudi Arabia was founded in 1932 by Ibn Saud. He united the four regions into a single state through a series of conquests beginning in 1902 with the capture of Riyadh, the ancestral home of his family, the House of Saud. Saudi Arabia has since been an absolute monarchy, effectively a hereditary dictatorship governed along Islamic lines. The ultraconservative Wahhabi religious movement within Sunni Islam. Saudi Arabia is sometimes called 'the Land of the Two Holy Mosques. in reference to Al-Masjid al-Haram (in Mecca) and Al-Masjid an-Nabawi (in Medina), the two holiest places in Islam. As of 2013, the state had a total population of 28.7 million, of which 20 million were Saudi nationals and 8 million were foreigners. As of 2017, the population is 33 million. The state's official language is Arabic. Petroleum was discovered on 3 March 1938 and followed up by several other finds in the Eastern Province. Saudi Arabia has since become the world's largest oil producer and exporter, controlling the world's second largest oil reserves and the sixth largest gas reserves. The kingdom is categorized as a World Bank high-income economy with a high Human Development Index and is the only Arab country to be part of the G-20 major economies. However, the economy of Saudi Arabia is the least diversified in the Gulf Cooperation Council, lacking any significant service or production sector (apart from the extraction of resources). The state has attracted criticism for its treatment of women and use of capital punishment. Saudi Arabia is a monarchical autocracy, has the fourth highest military expenditure in the world and SIPRI found that Saudi Arabia was the world's second