1+2-3+4+5-6+7+8-9.+97+98-99
=(1+2-3)+(4+5-6)+(7+8-9)+……+(97+98-99)
=0+3+6+9+……+96
=3*(1+2+3+4+……+32)
=3*(1+32)*32÷2
=1584