$2.$ Write python program for the Caesar cipher with the following requirements:

$(1)$ Define a function to encrypt with the following header. It takes in an English message $($containing $26$ letters only$)$ and a key between $0$ and $25$ inclusively, and returns the ciphertext:

def encrypt$($message$,$ key$)$:

$(2)$ Define a function to decrypt with the following header. It takes in a ciphertext and a key between $0$ and $25$ inclusively, and returns the plaintext:

def decrypt$($ciphertext$,$ key$)$:

$(3)$ Define a main$()$ function. First, it asks user what to do: encrypt or decrypt. Once the user makes the choice, it then asks for the text and the key. It should output the corresponding results.

def main$()$: