A solution for cs50's "caesar" problem: using a simple Caesar cipher

Simple Caesar

/**
 * caesar.c
 *
 * Peter Downs
 *
 * A code for encrypting text with a simple Caesar cipher.
 */

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

// Get the key to the cipher
int main(int argc, string argv[])
{
    string key = argv[1];
    if (argc != 2)
    {
        printf("Please give me a valid key");
        return 1;
    }
    else
    {
        // Convert the string to an integer
        int k = atoi(key) % 26;
        
        // Check that it is a valid key
        if (k == 0)
        {
            printf("Invalid key. Try again\n");
            return 1;
        }
    
        // Get the text to encrypt
        string plntxt = GetString();
        if (plntxt != NULL)
        {
    
            // Encrypt and print
            for (int i = 0, n = strlen(plntxt); i < n; i++)
            {
                int c = 0;    
        
                // Check if text is upper case or lower case
                // If so, switch to alphabet number and encrypt
                if (isupper(plntxt[i]))
                {
                   
                    c = (((int)plntxt[i] - 65 + k) % 26) + 65;
                    printf("%c", (char) c);
                }
                else if (islower(plntxt[i]))
                {
                    
                    c = (((int)plntxt[i] - 97 + k) % 26) + 97;
                    printf("%c", (char) c);
                }
                else
                {
                    printf("%c", plntxt[i]);
                }
            }
            printf("\n");
            return 0;
        }
    }
}