LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ '


def letter_to_num(c: str) -> int:
    """Return the number that corresponds to the given letter.

    Preconditions:
        - len(c) == 1 and c in LETTERS
    """
    return str.index(LETTERS, c)


def num_to_letter(n: int) -> str:
    """Return the letter that corresponds to the given number.

    Precondtions:
        - 0 <= n < len(LETTERS)
    """
    return LETTERS[n]


def encrypt_caesar(k: int, plaintext: str) -> str:
    """Return the encrypted message using the Caesar cipher with key k.

    Preconditions:
        - all({x in LETTERS for x in plaintext})
        - 1 <= k <= 26
    """
    ciphertext = ''

    for letter in plaintext:
        ciphertext = ciphertext + num_to_letter((letter_to_num(letter) + k) % len(LETTERS))

    return ciphertext


def decrypt_caesar(k: int, ciphertext: str) -> str:
    """Return the decrypted message using the Caesar cipher with key k.

    Preconditions:
        - all({x in LETTERS for x in ciphertext})
        - 1 <= k <= 26
    """
    plaintext = ''

    for letter in ciphertext:
        plaintext = plaintext + num_to_letter((letter_to_num(letter) - k) % len(LETTERS))

    return plaintext