How to make a caesar cipher program in C that will work for ANY ciper key/ shift entered, including negative keys

-reads characters from its input and writes the characters to its output encrypted with a Caesar cipher.

The number of positions to shift will be given to your program as a command line argument.

Characters other than letters should not be encrypted.

Your program should stop only at the end of input.

Your program should contain at least one function other than main.

Hint: handle upper and lower case letters separately

Hint: use %

Hint: use atoi to convert the first command-line argument to an int.