The 1970s marked a time of economic uncertainty and troubled relations between the U.S. and certain Middle-Eastern countries. To begin with, the decade started off with the ongoing Recession of 1969–70. Following that, the 1970s energy crisis ensued which included the 1973–75 recession, the 1973 oil crisis as well as the 1979 energy crisis beginning as a prelude to a disastrous economic climate injected with stagflation; the combination between high unemployment and high inflation. However, on November 14, 1972, the average closed above the 1,000 mark (1,003.16) for the first time, during a brief relief rally in the midst of a lengthy bear market. Between January 1973 and December 1974, the average lost 48% of its value in what became known as the 1973–1974 stock market crash; with the situation being exacerbated by the events surrounding the Yom Kippur War. The index closed at 577.60, on December 4, 1974. During 1976, the index went above 1000 several times, and it closed the year at 1,004.75. Although the Vietnam War ended in 1975, new tensions arose towards Iran surrounding the Iranian Revolution in 1979. Other notable disturbances such as the Lebanese Civil War, the Ethiopian Civil War, the Indo-Pakistani War of 1971 and the Angolan Civil War which the U.S. and Soviet Union considered critical to the global balance of power, seemed to have had little influence towards the financial markets. Performance-wise for the decade, gains remained virtually flat, rising less than 5% from about the 800 level to 838.
Question: How many years after the Vietnam War ended did the Iranian Revolution begin?
Answer: 4