def get_nth_subkeys_letters(nth, key_length, message):
    message = NONLETTERS_PATTERN.sub('', message)
    i = (nth - 1)
    letters = []
    while (i < len(message)):
        letters.append(message[i])
        i += key_length
    return ''.join(letters)
def get_nth_subkeys_letters(nth, key_length, message):
    message = NONLETTERS_PATTERN.sub(, message)
    i = (nth - 1)
    letters = []
    while (i < len(message)):
        letters.append(message[i])
        i += key_length
    return .join(letters)