def pi(N):

    ' 计算pi的值 '

    iter_1 = itertools.count(1,2)

    iter_2 = itertools.cycle([2,0])

    sum = 0

    for i in range(0,N):

        x,y = iter_1.__next__(),iter_2.__next__()

        sum += (-1 + y) * 4 / x

    return sum