#Key Generation
key = int(input("What key would you like to use? Enter a number between 1 and 25"))

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 = (input("Enter 'e' to encrypt or 'd' to decrypt"))
#default mode is e
mode = 'e'

if mode == 'd':
  key = -key
plain_text =(input("Enter data"))
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)