def pi(N):
    ' 计算pi的值 '
    # step 1: 创建一个奇数序列: 1, 3, 5, 7, 9, ...
    odds = itertools.count(start=1, step=2)

    # step 2: 取该序列的前N项: 1, 3, 5, 7, 9, ..., 2*N-1.
    os = itertools.takewhile(lambda x: (x+1) / 2 <= N, odds)

    # step 3: 添加正负符号并用4除: 4/1, -4/3, 4/5, -4/7, 4/9, ... 
    L = list(map(lambda x: (-1 if ((x+1)/2)%2==0 else 1) * 4 / x,list(os)))

    # step 4: 求和
    return sum(L)