再学python3：基础编程

1. 计算 1-2+3-4+5 ……-99 除去88的序列的和。符号一正一负 

  输出-50

a = 2
sum = 1
i = 1
while a < 100:
    if a < 88:
        if a % 2 == 0:
            sum -= a
        else:
            sum += a
    if a > 88:
        if a % 2 == 1:
            sum -= a
        else:
            sum += a
    a += 1

print(sum)