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

    Preconditions:
        - all({ord(c) < 128 for c in ciphertext})
        - 1 <= k <= 127

    >>> decrypt_ascii(4, 'Kssh$qsvrmrk%')
    'Good morning!'
    """
    plaintext = ''

    for letter in ciphertext:
        plaintext += chr((ord(letter) - k) % 128)

    return plaintext