CS50 caesar cypher

//caesar_character is a function that takes a single character and the key to use for the cypher and returns the encoded character back.

#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char caesar_character(char ch, int key);

int main(int argc, string argv[])
{
    if (argc != 2)
    {
        printf("You must suply exactly one argument!\n");
        return 1;
    }

    int key = atoi(argv[1]);
    string original = GetString();

    for (int i = 0, len = strlen(original); i < len; ++i)
    {
        printf("%c", caesar_character(original[i], key));
    }

    printf("\n");

    return 0;
}

char caesar_character(char ch, int key)
{
    if (islower(ch))
    {
        return (ch - 'a' + key) % 26 + 'a';
    }
    else if (isupper(ch))
    {
        return (ch - 'A' + key) % 26 + 'A';
    }
    else
    {
        return ch;
    }
}