Task: Question Generation
Input (English): Passage: 1st Squadron, 10th Cavalry  with the 4th Infantry Division participated in Exercise Reforger in 1977, 1978, 1981, 1985, 1987, and 1991. Exercise Reforger  was an annual exercise conducted, during the Cold War, by NATO. The exercise was intended to ensure that NATO had the ability to quickly deploy forces to West Germany in the event of a conflict with the Warsaw Pact. 2nd Squadron, 10th Cavalry  with the 7th Infantry Division participated in Exercise Reforger in 1984, 1986 and 1993. 3rd Battalion, 10th Cavalry  was activated in the 1st Cavalry Division at Fort Hood, Texas, in 1977.
Output (English): How many years was it from the time 1st Cavalry Division was activated until the fourth time they participated in the Extercise Reforger?