def shift_string(string, offset):
    alphabet = "abcdefghijklmnopqrstuvwxyz"
    max_offset = len(alphabet) -1
    min_offset = 0

    output = ""

    for letter in string.lower():
        letter_index = alphabet.find(letter)
        if letter_index == -1:
            result = letter
        else:
            if letter_index + offset > max_offset:
                result = alphabet[letter_index - len(alphabet) + offset]
            elif letter_index + offset < 0:
                result = alphabet[letter_index + len(alphabet) + offset]
            else:
                result = alphabet[letter_index + offset]

        output += result
    
    print(f"Your result is: {output}")


b_continue = "yes"

while b_continue == "yes":
    action = input("What do you want to do? \"encode\" or \"decode\"?\n")
    message = input("Type your message.\n")
    offset = int(input("What is your offset?\n"))

    if action == "encode":
        shift_string(message, offset)
    elif action == "decode":
        shift_string(message, -offset)
    else:
        print("Invalid action - Try again")

    b_continue = input("Do you want to continue? \"yes\" or \"no\"\n")