Need help with RelativeSource Syntax

I have ListView which has a ListView.Header. The ListView is bound to an ObservableCollection. The Header needs to bind to members of my view model, not the collection. I cant figure out the correct syntax:

view model

    public class MilitaryAssestsDetailsViewModel 
    {
        public int Armies 
        {
            get { return GetArmies(); }
        }

        public ObservableCollection<TerritoryModel> TerritoryList
        {
            get { return _gameData.PlayerLand.Territories; }
        }

    }

xaml:

    <StackLayout>
<ListView ItemsSource="{Binding TerritoryList}" >
        <ListView.Header>
            <StackLayout>
                <Label Text="Your Military:" />
                <StackLayout Orientation="Horizontal">
                    <Label Text="{Binding Path=Armies, StringFormat='{0} Armies'}}" />
                </StackLayout>                          
            </StackLayout>
        </ListView.Header>  

        <ListView.ItemTemplate>
        </ListView.ItemTemplate>
    </ListView>
</StackLayout>

The binding to "TerritoryList" works as a list appears. But the content for the header does not work.

I've tried:

<Label Text="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type StackLayout}}, Path=Armies}" />

but it throws an exception that RelativeSource markup extension doesn't exist.

Thnx

Matt