def substr(string: str, start: int, length: int | None = None) -> str:

    """
    Find Substring of a original String.
    :param string: Original String.
    :param start: Start of Substring.
    :param length: Length of Substring.
    :return: Substring of original String.
    """

    if length is not None and (length < 0 or length > len(string)):
        raise ValueError("invalid Param")
    if length is None:
        return string [start:]
    else:
        return string[start:start+length]

print(substr("GEEKSFORGEEKS", 0,5))
print(substr("GEEKSFORGEEKS", 4, 2))