def pi(N):

    natuals = itertools.count(1, 2)

    ns = itertools.takewhile(lambda x: x < 2 * N, natuals)

    items = []

    list1 = list(ns)

    for i in range(len(list1)):

        if (i + 1) % 2 == 0:

            items.append(1 / -list1[i])

        else:

            items.append(1 / list1[i])

    return sum(items) * 4