from collections import deque

def pi(N):

    # step 1: 创建一个奇数序列: 1, 3, 5, 7, 9, ...

    arrayOdd = itertools.count(1, 2)

    # step 2: 取该序列的前N项: 1, 3, 5, 7, 9, ..., 2*N-1.

    arrayOddN = deque()

    for i in range(N):

        arrayOddN.append(next(arrayOdd))

    # step 3: 添加正负符号并用4除: 4/1, -4/3, 4/5, -4/7, 4/9, ...

    arrayFinal = [4/x if i%2==0 else -4/x for i, x in enumerate(arrayOddN)]

    # step 4: 求和:

    return sum(arrayFinal)