#!/usr/bin/env python3
# _*_ coding:utf-8 _*_

__author__ = 'chengy'
'doctest 文档测试 '

def fact(n):
    '''
    Calculate 1*2*3*...*n
    >>> fact(1)
    1
    >>> fact(10)
    ?
    >>> fact(-1)
    ?
    '''

    if n < 1:
        raise ValueError
    if n == 1:
        return 1
    return n * fact(n - 1)


if __name__ == '__main__':
    import doctest

    doctest.testmod()