Rondônia is one of the 26 states of Brazil, located in the northern subdivision of the country between latitudes 07º 58' 32" and 13º 41' 36" S and longitudes 59º 46' 26" and 66º 48' 21"W. To the west is a short border with the state of Acre for 22 km, to the north is the state of Amazonas for about 930 km across the Serra de São Jão, latitude 7° 58ˈ south, and north of the Mapiri-Madeira River, in the east is Mato Grosso for about 820 km across the Serra Grande, Serra de Providência and the Rio Branco, Inue (Languiaru) and Tenente Marques-Eugênia, and in the south and southwest is Bolivia for 1,464 km across the Guaporé and Abuna Rivers. Rondônia has a population of 1,815,000 as of 2021. It is the fifth least populated state. Its capital and largest city is Porto Velho, bathed by the Madeira River. The state was named after Cândido Rondon, who explored the north of the country during the 1910s. The state, which is home to c. 0.7% of the Brazilian population, is responsible for c. 0.3% of the Brazilian GDP.