Android - Confirm app exit with toast

I'm new to Android development and I want it so when the user presses the back button on the main activity, a toast message appears with a "confirm exit by pressing the back button again" message. How would I do this? This is what I have so far:

@Override
public void onBackPressed() {
    // TODO Auto-generated method stub
    super.onBackPressed();
    Toast s = Toast.makeText(getBaseContext(), "Press back again to exit", Toast.LENGTH_LONG);
    s.show();
    wait();

    public boolean onBackPressed() {
        finish();    
    }
}