Tajikistan (i/tɑːˈdʒiːkᵻstɑːn/, /təˈdʒiːkᵻstæn/, or /tæˈdʒiːkiːstæn/; Persian: تاجيكستان‎‎ Тоҷикистон [tɔd͡ʒikɪsˈtɔn]), officially the Republic of Tajikistan (Persian: جمهورى تاجيكستان‎‎ Tajik: Ҷумҳурии Тоҷикистон, Çumhuriji Toçikiston/Jumhuriyi Tojikiston; Russian: Респу́блика Таджикистан, Respublika Tadzhikistan), is a mountainous, landlocked country in Central Asia with an estimated 8 million people in 2013, and an area of 143,100 km2 (55,300 sq mi). Tajikistan had an estimated 18 million people in what year?