1+2-3-4+5+6-7-8+9+10-11-12+13.-1983-1984+1985+1986
=1+[(2-3)+(-4+5)]+[(6-7)+(-8+9)]+[(10-11)+(-12+13)].-1983)+(-1984+1985)]+1986
=1+0+0+0.+0+1986
=1987