chars = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']
NUMBER_CHARACTERS = len(chars)


def encrypt(message, multiple):
    encrypted = ""
    substitute = []

    for i in range(NUMBER_CHARACTERS):
        idx = (i * multiple) % NUMBER_CHARACTERS
        substitute.append(chars[idx])

    for i in range(len(message)):
      char = message[i].lower()
      if char in chars:
            idx = chars.index(char)
            encrypted += substitute[idx]
      else:
            encrypted+= message[i]

    return encrypted

def decrypt(message, multiple):
    decrypted = ""
    encrypted = ""
    substitute = []
    msg = encrypted
    msg = msg.lower()

    for letter in msg:
        if letter in substitute:
         idx = substitute.index(letter)
         decrypted += chars[idx]

        else:
         decrypted += letter

    return decrypted


def main():
    choose = input("Would you like to encrypt or decrypt a message? ")

    if choose == "encrypt":
        text = input("Enter the message: ")
        multiple = int(input("Enter the multiple value: "))
        result = encrypt(text, multiple)
        print("Your encrypted message:" ,result)

    elif choose == "decrypt":
        text = input("Enter the message: ")
        multiple = int(input("Enter the multiple value: "))
        result = decrypt(text, multiple)
        print("Your decrypted message:" ,result)

    else:
        print("Invalid choice. Please enter 'encrypt' or 'decrypt'.")
main()