Php – Codeigniter extends core class

I want to have class that checks login on all controllers that I specified.
Codeigniter version is 2.1.0 and I have php 5.3.10

Hier is how I would set it up:
I look at https://www.codeigniter.com/user_guide/general/core_classes.html
and I set it up like this:
in the /application/core/MY_Main.php

class MY_Main extends CI_Controller {

function __construct()
{
    parent::__construct();
}
}

In my controller I have welcome.php

?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class Welcome extends MY_Main {

    function __construct()
    {
            parent::__construct();
    }
    public function index()
    {
            $this->load->view('welcome_message');
    }
}

So if I place login check in MY-Main it should get work, but I cant get it work anyone???