Python求1!-2!+3!-4!……-20!的和

Python求1!-2!+3!-4!+5!-6!……-20!的和

#阶乘函数
def fn(n):
    if n == 1:
        return n
    else:
        return n*fn(n-1)
 
n = 20
s = 0
flag = 1
for i in range(1,n+1):
    s = s + flag*fn(i)
    flag = -flag
print(s)