def fact(n):
    '''
    
    Calculate 1*2*...*n
    
    >>> fact(1)
    1
    >>> fact(10)
    3628800
    >>> fact(-1)
    Traceback (most recent call last):
          ...
    ValueError
    '''
    if n < 1:
       raise ValueError()
    if n == 1:
       return 1
    return n* fact(n-1)

if __name__ == '__main__':
    import doctest
    doctest.testmod()