key = 'abcdefghijklmnopqrstuvwxyz'
def dec_caesar(n, ciphertext):
    result = ''
    for l in ciphertext:
        try:
            i = (key.index(l) - n) % 26
            result += key[i]
        except ValueError:
            result += l
    return result

ciphertext = 'xzdcdpb qdvwhsxmdfh sudzd cd rfcbzlvwh: zvcbvfb oxgclh vwzruchql vd urzqbpl'
plaintext = dec_caesar(3, ciphertext)
print(plaintext)