def get_natural_num(msg):

    """
    Get a valid natural number from the user!

       :param msg: message asking for a natural number
       :return: a positive integer converted from the user enter.
       """

    valid_enter = False

    while not valid_enter:

        given_number = input(msg).strip()

        if given_number.isdigit():
            num = int(given_number)
            valid_enter = True
            return num