def pi(N):

    ns = itertools.count(1, 2)

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

    sum=0

    L=list(nc)

    a=[-4/L[i] if i%2 else 4/L[i] for i in range(N)]

    for i in a:

      sum+=i  

    return sum