How to skip a page in back navigation in windows Phone 7

Question

Hi, I am developing a application on windows phone 7. So my landing page is a custom Splash Screen page ( i didn't use the default way of showing a splash screen ie. having a png file, as i need to add data in the splash screen at runtime based on the user profile). So i created a splashscreen page. After a period of delay ( 5 sec), I navigate to my main page. using

PhoneApplicationFrame root = Application.Current.RootVisual as PhoneApplicationFrame;
 root.Navigate(new Uri("/MainPage.xaml", UriKind.Relative));

Now, if the user presses the back button of the phone, by default to goes to the splash screen, but i want the application to exit.

How i can achive this. .