求1-2+3-4+5 ... 99的所有数的和

a=1
b=0
while a<101:
    if a%2==1:
        b=a+b
    a=a+1
c=1
d=0
while c<101:
    if c%2==0:
        d=d-c
    c=c+1
print(b+d)