Between 1951 and 1973, Afghanistan's year-end international reserves were never lower than $38 million or higher than $65 million. Development of the natural gas industry and favorable prices for some of the country's agricultural exports led to increases in international reserves, to $67.5 million in 1974 and to $115.4 million as of 31 December 1975. Exploitation of natural gas also freed Afghanistan from extreme dependence on petroleum imports and from the rapid increases in import costs that most countries experienced in 1973 and 1974. Increased trade with the former USSR and Eastern Europe in the late 1970s and 1980s resulted in a reduction of foreign exchange earnings, since trade surpluses are counted as a credit against future imports. Foreign exchange reserves declined from $411.1 million at the close of 1979 to $262 million as of 30 May 1987. The public foreign debt in 1997 stood at $5.49 billion. Reliable statistics are not available for the ensuing years. However, the US Central Intelligence Agency (CIA) reports that in 2001 the purchasing power parity of Afghanistan's exports was $1.2 billion while imports totaled $1.3 billion resulting in a trade deficit of $100 million.