def fib(n):
    """ This function calculate fib number.

    Example:

        >>> fib(10)
        55
        >>> fib(-1)
        Traceback (most recent call last):
            ...
        ValueError
    """
    if n < 0:
        raise ValueError('')
    return 1 if n<=2 else fib(n-1) + fib(n-2)