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

int main(int argc, string argv[])
{
    string k = argv[1];
    if (argc == 2 && isalpha(k))
    {
        goto cypher;
    }
    else
    {
        printf("Wrong command-line argument! Try again!\n");
        return 1;
    }
    cypher: printf("Please enter massage you want to cypher!\n");
    string text = GetString();
    for (int i =0, n = strlen(text), m = strlen(k); i < n; i++)
    {
         if (isalpha(text[i]))
         {
             if (isupper(text[i]))
             {
             char letter = ((text[i] + k[i % m]) - 65) % 26;
             printf("%c", letter + 65);
             }
             else
             {
             char letterlow = ((text[i] + k[i % m]) - 97) % 26;
             printf("%c", letterlow + 97);
             }
         }
         else
         {
             printf("%c", text[i]);
         }
    }
    printf("\n");
}