Caesar's Cipher

def convert(message, offset):
    alphabet = "abcdefghijklmnopqrstuvwxyz"
    result =""
    for i in range(len(message)):
        index = -1000
        for j in range(len(alphabet)):
            if message[i].lower() == alphabet[j]:
                index = j

        if index == -1000:
            result += message[i]
        else:
            newIndex = index + offset
            if message[i] == message[i].lower():
                result += alphabet[newIndex % len(alphabet)]
            else:
                result += alphabet[newIndex % len(alphabet)].upper()
    return result

print(convert("HelloOOO WORLDdd", -26))