Answer based on context:

With no official figures available, it is estimated that 600,000-900,000 persons fled the country during the Lebanese Civil War . Although some have since returned, this permanently disturbed Lebanese population growth and greatly complicated demographic statistics. Another result of the war was a large number of internally displaced persons. This especially affected the southern Shia community, as Israeli invasion of southern Lebanon in 1978, 1982 and 1996 prompted waves of mass emigration, in addition to the continual strain of occupation and fighting between Israel and Hezbollah . Many Shias from Southern Lebanon resettled in the suburbs south of Beirut. After the war, the pace of Christian emigration accelerated, as many Christians felt discriminated against in a Lebanon under increasingly oppressive Syrian occupation. According to a UNDP study, as much as 10% of the Lebanese had a disability in 1990. Other studies have pointed to the fact that this portion of society is highly marginalized due to the lack of educational and governmental support of their advancement.

How many years passed between the first and the third Israeli invasions in Lebanon?
18