SetCancelable and Back Button (android)

I have an app and When the user presses BACK Button, it shows dialog, the dialog cannot be cancelled so I set it setCancelable(false)

but I want to dismiss the dialog when the user presses BACK button Again. ( if the user presses Back Button, Show dialog. if the user presses Back Button again , dismiss dialog )

My Code below

private boolean dialogshown = false;
private AlertDialog d;
public boolean onKeyDown(int keycode, KeyEvent e) {   
  if(keycode == KeyEvent.KEYCODE_BACK)
    if(!dialogshown) {
        d = new AlertDialog.Builder(MainActivity.this).create();
        d.setButton(AlertDialog.BUTTON_POSITIVE, "Cancel",
                new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        dialog.dismiss();
                        dialogshown = false;
                    }
                });
        d.setButton(AlertDialog.BUTTON_NEGATIVE, "Exit",
                new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        finish();
                        System.exit(0);
                    }
                });
        d.setCancelable(false);
        d.show();
        dialogshown = true;
    } else {
        try{ d.dismiss(); }catch (Exception exc){Toast.makeText(this,"Error back",Toast.LENGTH_LONG).show();}
    }
return super.onKeyDown(keycode, e);
}

The problem is when I press Back button it show a dialog, but when I press it again Nothing Happens