(Cs50) Caesar

caesar.c

#include <stdio.h>
#include <stdlib.h>
#include <cs50.h>
#include <string.h>
#include <ctype.h>
int main(int input, string argv[])
{
    if(input == 2)
    {
        int key = atoi(argv[1]);
        if  (key > 0)
        {
            string plaintext = GetString();
            for (int currentchar = 0, n = strlen(plaintext); currentchar < n; currentchar++)
            {
                if(isalpha(plaintext[currentchar]))
                {
                int a = plaintext[currentchar];
                char b;
                    if (isupper(a)) {b = 'A';}
                    else {b = 'a';}
                    plaintext[currentchar] = ((((plaintext[currentchar] + key)- b) % 26)+b);
                }
                printf("%c",plaintext[currentchar]);
            }
            printf("\n");
        }
        else 
        {
            printf("<key> need to be a positive integer\n");
            return 1;
        }
    }
    else
    {
        printf("Invalid parameter !: caesar <key>\n");
        return 1;
    }
}