def convert_letter_to_number(letter: str) -> int:
    """
    This function receives a string representing a letter of the alphabet, and convert it to
    its respective number in ascending order, for example:
    A - 1
    B - 2
    C - 3
    D - 4

    :param str letter: The letter to be converted.
    :rtype: int
    """
    return [ord(char) - 96 for char in letter.lower()][0] - 1