def fact(n):
    """
    >>> fact(-5)
    Traceback (most recent call last):
          File "<pyshell#106>", line 1, in <module>
            fact(-5)
          File "<pyshell#104>", line 8, in fact
            raise ValueError()
        ValueError
    >>> fact(1)
    1
    >>> fact(5)
    120
    """
    if n < 1:
        raise ValueError()
    if n == 1:
        return 1
    return n * fact(n - 1)


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