Caesar

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

int main(int argc, string argv[])
{
    int KEY = 0;
    bool NUM = true;
    int j = 0;
    //checks if there are 2 args entered
    if (argc == 2)
    {
        j = strlen(argv[1]);
    }
    else
    {
        //explains useage if argv1 was not correctly entered
        printf("Usage: ./cesar key\n");
        return 1;
    }
    //checks if argv[1] is a number
    for (int i = 0; i < j; i++)
    {
        if (isdigit(argv[1][i]))
        {
            NUM = true;
        }
        else
        {
            NUM = false;
            break;
        }
    }
    //only runs main program if all checks above are true (argv[1] is a number and there is only 2 args entered)
    if (NUM == true)
    {
        string plain = (get_string("Enter your plaintext: \n"));
        //converts argv[1] to int
        for (int i = 0, n = strlen(argv[1]); i < n; i++)
        {
            KEY = KEY * 10 + (int) argv[1][i] - '0';
        }
        //reduces the key smallest necessary integer. Over 26 loops around extra.
        while (KEY > 26)
        {
            KEY = KEY - 26;
        }
        //iterates over the length of the plaintext string
        for (int i = 0, n = strlen(plain); i < n; i++)
        {
            //checks if the character is uppercase
            if (plain[i] >= 'A' && plain[i] <= 'Z')
            {
                //checks that the character plain[i] + the key remains within the uppercase letters in ASCII
                char c = plain[i] + KEY;
                if (c >= 'A' && c <= 'Z')
                {
                    printf("%c", c);
                }
                //if plain[i] + key is higher than 'Z' it will remove '26' to bring it back within confines of A through Z
                else
                {
                    c = c - 26;
                    printf("%c", c);
                }
            }
            //checks if the character plain[i] is a lowercase letter
            else if (plain[i] >= 'a' && plain[i] <= 'z')
            {
                //checks that the character plain[i] + the key remains within the lowercase letters in ASCII
                char c = plain[i] + KEY;
                if (c >= 'a' && c <= 'z')
                {
                    printf("%c", c);
                }
                else
                {
                    //if plain[i] + key is higher than 'z' it will remove '26' to bring it back within confines of a through z
                    c = c - 26;
                    printf("%c", c);
                }
            }
            //if character is not a letter, just prints the character as is
            else
            {
                printf("%c", plain[i]);
            }
        }
        //writes a new line for cleanliness at the end
        printf("\n");
        return 0;
    }
    else
    {
        //explains useage if argv1 was not correctly entered
        printf("Usage: ./cesar key\n");
        return 1;
    }
}