Custom Total in Matrix

I have a row group, inside this row group I wish to subtract Revenue - Expenses. Right now it is just simply adding them all up.

Desired:

Any ideas?

You can see I am already sorting the row group so 'Revenue' comes first and all 'Expense' types come after. I am doing this using a sort expression:

=Switch(Fields!LedgerTypeCode.Value="IN","1",
Fields!LedgerTypeCode.Value="SE", "2",
Fields!LedgerTypeCode.Value="CE", "3",
Fields!LedgerTypeCode.Value="OE", "4",
Fields!LedgerTypeCode.Value="AE", "5")

Craig