from typing import List

def substring(string: str, start: int, letters: int =None) -> str:
    """
    creates a substring from a given string
    :param string: the original string
    :param start: the start index
    :param letters: the amount of letters in the substring
    :return: the substring
    """
    length = len(string)
    if start > length:
        raise ValueError ("Start index must be smaller than stringlenght")
    if letters is None:
        letters = length - start
    if letters + start > length:
        raise ValueError ("Sum of start and numbers of letters is grater that stringlength")
    if start <0:
        raise ValueError (" Start must be positive")
    return string[start: start+ letters]

if __name__ == "__main__":
    original = "GEEKSFORGEEKS"
    print(substring(original, 0, 5))  # Output: GEEKS
    print(substring(original, 5))  # Output: FORGEEKS


def product (numbers:List)-> int:
    temp = 1
    result = 1
    for i in numbers:
        result *= i
        i += 1
    return result

if __name__ == "__main__":
    print(product([1,2,3]))