Detailed Instructions: This task involves creating answers to complex questions, from a given passage. Answering these questions, typically involve understanding multiple sentences. Make sure that your answer has the same type as the "answer type" mentioned in input. The provided "answer type" can be of any of the following types: "span", "date", "number". A "span" answer is a continuous phrase taken directly from the passage or question. You can directly copy-paste the text from the passage or the question for span type answers. If you find multiple spans, please add them all as a comma separated list. Please restrict each span to five words. A "number" type answer can include a digit specifying an actual value. For "date" type answers, use DD MM YYYY format e.g. 11 Jan 1992. If full date is not available in the passage you can write partial date such as 1992 or Jan 1992. If you find multiple spans, please add them all as a comma separated list. Please restrict each span to five words.
Q: Passage: As Somalia gained military strength, Ethiopia grew weaker. In September 1974, Emperor Haile Selassie had been overthrown by the Derg , marking a period of turmoil. The Derg quickly fell into internal conflict to determine who would have primacy. Meanwhile, various anti-Derg as well as separatist movements began throughout the country. The regional balance of power now favoured Somalia. One of the separatist groups seeking to take advantage of the chaos was the pro-Somalia Western Somali Liberation Front operating in the Somali-inhabited Ogaden area, which by late 1975 had struck numerous government outposts. From 1976 to 1977, Somalia supplied arms and other aid to the WSLF. A sign that order had been restored among the Derg was the announcement of Mengistu Haile Mariam as head of state on February 11, 1977. However, the country remained in chaos as the military attempted to suppress its civilian opponents in a period known as the Red Terror . Despite the violence, the Soviet Union, which had been closely observing developments, came to believe that Ethiopia was developing into a genuine Marxist-Leninist state and that it was in Soviet interests to aid the new regime. They thus secretly approached Mengistu with offers of aid that he accepted. Ethiopia closed the U.S. military mission and the communications centre in April 1977. In June 1977, Mengistu accused Somalia of infiltrating SNA soldiers into the Somali area to fight alongside the WSLF. Despite considerable evidence to the contrary, Barre strongly denied this, saying SNA "volunteers" were being allowed to help the WSLF.
Question: How many years passed between Emperor Haile Selassie being overthrown and the Derg announcing the new head of state?
Answer type: Number
A:
3.