Back navigation of pages

Question

Have a List if Region. On selecting a region it shoe list of countries on that region on another page.selecting a country from next page it shows the details of that country on another page.how can i navigate back to each page???

public sealed partial class MainPage : Page
{
public MainPage()
{
this.InitializeComponent();

DataContext = new MainViewModel();
}
protected override void OnNavigatedTo(NavigationEventArgs e)
{

}

private void ListView_ItemClick(object sender, ItemClickEventArgs e)
{
WorkItem i = (e.ClickedItem as WorkItem);
Frame.Navigate(typeof(Country), i);
}
}

country_List.xaml.cs 

public sealed partial class Country : Page
   {
       public Country()
       {
           this.InitializeComponent();
       }


       protected override void OnNavigatedTo(NavigationEventArgs e)
       {
           WorkItem Name = e.Parameter as WorkItem;
           base.OnNavigatedTo(e);
           this.DataContext = new CountryViewModel(Name);
       }

   }

 

 how can i navigate back to my main page...