Nepal remains isolated from the world’s major land, air and sea transport routes although, within the country, aviation is in a better state, with 47 airports, 11 of them with paved runways; flights are frequent and support a sizable traffic. The hilly and mountainous terrain in the northern two-thirds of the country has made the building of roads and other infrastructure difficult and expensive. In 2007 there were just over 10,142 km (6,302 mi) of paved roads, and 7,140 km (4,437 mi) of unpaved road, and one 59 km (37 mi) railway line in the south. There is a single reliable road route from India to the Kathmandu Valley. More than one-third of its people live at least a two hours walk from the nearest all-season road; 15 out of 75 district headquarters are not connected by road. In addition, some 60% of road network and most rural roads are not operable during the rainy season. The only practical seaport of entry for goods bound for Kathmandu is Calcutta in India. Internally, the poor state of development of the road system makes access to markets, schools, and health clinics a challenge.