def convert_number_to_letters(input : int) -> str:
    """ Convert numbers to letters, e.g. 1 to 'a' or 137 to 'eg' """

    def number_to_letter(number: int) -> str:
        if number == 0: return ''
        return chr(ord('`')+number)

    quotient, remainder = divmod(input,26)
    output = f'{number_to_letter(quotient)}{number_to_letter(remainder)}'

    return output