import itertools

def pi(N):

# step 1: 创建一个奇数序列: 1, 3, 5, 7, 9, ...
odd = itertools.count(1,2)

# step 2: 取该序列的前N项: 1, 3, 5, 7, 9, ..., 2*N-1.
oddn = itertools.takewhile(lambda x: x <= 2*N, odd)
L1 = list(oddn)

# step 3: 添加正负符号并用4除: 4/1, -4/3, 4/5, -4/7, 4/9, ...
for i in range(N):
        L1[i] = 4/L1[i]*((-1)**i)

# step 4: 求和:
return sum(L1)