Write a question about the following article: Iran's telecommunication network enjoys the highest growth rate in the Middle East. One of the indicators that clearly illustrates this growth rate is the increase in the number of installed main lines. In 1978 there were only about 850,000 fixed lines installed in Iran. This number rose to about 1,767,000 by 1986 and to about 2,380,000 by 1989. One of the objectives of the first five-year development plan (1989-1994) for the telecommunication sector was to add one million new fixed lines to the network. However, due to the establishment and expansion of many low-capacity and high-capacity exchanges, the actual growth rate that was achieved was considerably higher. In fact more than 3.1 million new main lines were installed during the period of the first five-year plan. Four million new lines were expected to be added by the end of the second five-year plan (1994-1999), which would increase the number of installed main lines to about 9,510,000 fixed lines. Irans telecom market, which generated an estimated $9.1 billion in 2010 (of which 2/3 from "voice" mobile-phone and 1/10 from "data" mobile-phone services), has been growing significantly, especially its mobile-phone business. As of 2010, Iran had about 66 million mobile-phone subscribers covering about 70% of the population. In contrast, about 36% of Iranians had fixed-line phones.

Question about the article:
Between 1978 and 1986, how many main lines were installed?