This code will allow the user to repeat the program until they input "end"
key = int(input("Enter the key you wish to use: "))
letter = input("Enter a letter you wish to encrypt / decrypt")
while letter != "end":
letter_number = ord(letter)
letter_number_add_key = letter_number + key
new_letter = chr(letter_number_add_key)
print("New Letter =")
print(new_letter)
letter = input("Enter another letter or type end: ")
print("Goodbye!")