Eu tenho um seletor de lista longa

<phone:LongListSelector x:Name="BTDevices"  SelectionChanged="BTDevices_SelectionChanged_1">
 <phone:LongListSelector.ItemTemplate>
  <DataTemplate>
   <StackPanel>
    <TextBlock Text="{Binding Path=Name}" FontSize="30" />
   </StackPanel>
  </DataTemplate>
 </phone:LongListSelector.ItemTemplate>
</phone:LongListSelector>

A função é definida como:

private void BTDevices_SelectionChanged_1(object sender, SelectionChangedEventArgs e)
{
//here i want to get the index of the selected item
}

Eu tentei a seguinte linha

int a = App.ViewModel.Items.IndexOf(sender as ItemViewModel);

Mas sempre retorna -1.