import random

#Key Generation
key = int(input("What key would you like to use? Enter a number between '1' and '25' or '0'to generate a random key\n"))

if key == 0:
  key = random.randint(1,25)
else:
  key = key

while key < 1 or key > 25:
  print("Invalid key")
  key = int(input("What key would you like to use? Enter a number between 1 and 25"))

print("Key Selected: " + str(key))

mode = int(input("Choose mode:\n 1 = Encrypt\n 2 = Decrypt\n"))

if mode == 2:
  key *= -1
else:
  key = key

print ("Key required:  " + str(key))

plain_text = input("Enter the text you wish to encrypt/decrypt: \n")
cipher_text = ""

for letter in plain_text:
    if letter.isalpha():
        val = ord(letter)
        val = val + key
        if letter.isupper():
            if val > ord('Z'):
                val -= 26
            elif val < ord('A'):
                val += 26
        elif letter.islower():
            if val > ord('z'):
                val -= 26
            elif val < ord('a'):
                val += 26
        new_letter = chr(val)
        cipher_text += new_letter
    else:
        cipher_text += letter
print(cipher_text)