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(1,2)
	# step 2: 取该序列的前N项: 1, 3, 5, 7, 9, ..., 2*N-1.
	ns = itertools.takewhile(lambda x: x <= (2 * N -1), odds)
	# step 3: 添加正负符号并用4除: 4/1, -4/3, 4/5, -4/7, 4/9, ...
	pm =list(ns)
	result = list(map(lambda x: float(4 / x * 1 if pm.index(x) % 2 == 0 else 4 / x * -1), pm))
	# step 4: 求和:
	return sum(result)