Caesar done!

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

int main(int argc, string argv[])
{    
    // Check if 2 commandline arguments are given, otherwise abort program
    if (argc == 2)
    {
        // Loop through argv[1] to check if only numeric inputs are given
        int n = strlen(argv[1]);
        for (int i = 0; i < n; i++)
        {
            if (isdigit(argv[1][i]))
            {
                printf ("Success\n");
            }
            else 
            {
                printf ("Usage: ./ceasar key\n");
                return 1;
            }
        }
    }
    else
    {
        printf ("Usage: ./ceasar key\n");
        return 1;
    };
    
    // Transform argv[1] to int and assign to key variable
    int key = atoi(argv[1])%26;
    printf ("%i\n", key);
    
    // Ask user for secret message
    string message = get_string("plaintext: ");
    // Determine length of secret message
    int length = strlen(message);
    // Print "ciphertext: " for layout reasons
    printf ("ciphertext: ");
    // Loop through message to print every char misplaced by 1 on same line
    for (int i = 0; i < length; i++)
    {
        // Retain uppercase
        if (message[i] >= 65 && message[i] <= 90)
        {
            char newlet = message[i] + key;
            if (newlet > 90)
            {
               printf ("%c", newlet - 26);
            }
            else
            {
                printf ("%c", newlet);
            }
        }
        // Retain lowercase
        else if (message[i] >= 97 && message[i] <= 122)
        {
            char newlet = message[i] + key;
            if (newlet > 122)
            {
               printf ("%c", newlet - 26);
            }
            else
            {
                printf ("%c", newlet);
            }
        }
        // Retain punctuation
        else
        {
          printf ("%c", message[i]);  
        }
    }
    // Go to next line
    printf ("\n");
    
}