elements left in stack

I want to check number of elements in my react navigation stack. if there was only one screen left and user press back button, application show them a message to exit the app. I had to use BackHandler in all of my screens because I hadn't had found a property which could show elements in stack.
this is my routes

const route1= {
	screen1,
	screen2
};

const route2= {
        screen3,
	screen4
};

export const stack1= createStackNavigator(route1);
export const stack2= createStackNavigator(route2);

export default {stack1, stack2};

this is my navigator

createNavigator = (routeName, params) => {
		this.navigator = createAppContainer(createSwitchNavigator(routes, { initialRouteName: routeName, initialRouteParams: params }));
	}

Inizia esaminando gli entry point createStackNavigator e createSwitchNavigator mostrati nell’issue, insieme al contesto di BackHandler di React Native. Determina se la documentazione debba spiegare come rilevare una sola schermata rimanente e gestire l’Exit-Prompt; il lavoro è completato quando l’approccio supportato è documentato con un esempio corrispondente alla configurazione stack1/stack2.