Laravel 5.2 - default error page for undefined routes

Suppose I have this in my routes.php :

Route::get('sells', ['as' => 'user_sells', 'uses' => 'SellsController@indexAll']);

Now, as you know, if someone opens mySite.com/sells, the desired method in the controller would be executed.

But, if someone tried to access to a route which is not defined at all, like mySite.com/buys , I want to show a default error page. I mean, I need to say if a route is not defined, show a specific page.

How can I do that?

Thanks in advance

via Chebli Mohamed