In 1976, the then Syrian president Hafez al-Assad sent troops into Lebanon to fight PLO forces on behalf of Christian militias. This led to escalated fighting until a cease-fire agreement later that year that allowed for the stationing of Syrian troops within Lebanon. The Syrian presence in Lebanon quickly changed sides; soon after they entered Lebanon they had flip-flopped and began to fight the Christian nationalists in Lebanon they allegedly entered the country to protect. The Kateab Party and the Lebanese Forces under Bachir Gemayel strongly resisted the Syrians in Lebanon. In 1989, 40,000 Syrian troops remained in central and eastern Lebanon under the supervision of the Syrian government. Although, the Taif Accord, established in the same year, called for the removal of Syrian troops and transfer of arms to the Lebanese army, the Syrian Army remained in Lebanon until the Lebanese Cedar Revolution in 2005 to end the Syrian occupation of Lebanon. In 1994, the Lebanese government under the pressure of the Syrian government, gave Lebanese passport to thousands of Syrians. There are nearly 1.08 million registered Syrian refugees in Lebanon.
Question: How many years was it from when Hafez al-Assad sent troops into Lebanon and when 40,000 Syrian troops remained in central and eastern Lebanon?
Answer: 13