ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"


def caesar_enc(letter: str) -> str:
    if not 'A' <= letter <= 'Z':
        raise ValueError('Invalid letter')
    return ALPHABET[(ord(letter) - ord('A') + 3) % len(ALPHABET)]


def caesar_enc_string(plaintext: str) -> str:
    ciphertext = ""
    for letter in plaintext:
        ciphertext += caesar_enc(letter)
    return ciphertext


def main() -> None:
    m = "BINEATIVENIT"
    c = caesar_enc_string(m)
    print(c)


if __name__ == "__main__":
    main()