int main(int argc, string argv[])
{
    int e = 0;
    string k;
    string p;
    int key;

    if (argc == 2)
    {
        k = argv[1];
    }
    else
    {
        e++;
        printf("Error: %in", e);
    }
    key = atoi(argv[1]);

    p = get_string("plaintext: ");

    for (int i = 0, n = strlen(p); i < n; i++)
    {
        if (p[n] >= 'a' && p[n] <= 'z')
        {
            p[n] = (p[i] + key - 'a') % 26 + 'a';
        }
        else if (p[n] >= 'A' && p[n] <= 'Z')
        {
            p[n] = (p[i] + key - 'A') % 26 + 'A';
        }
    }
    printf("ciphertext: %sn", p);
    return 0;
}