SUM in ReportViewer

Hi, 

I have a reportviewer control in my asp .net page. In the rdlc file, I am creating a table. It looks something like this :

                                  ItemName       Rate          Qty          Discount          Amount

                  Table Rows   [ItemName]      [Rate]      [Qty]            [Disc]              [ItemCost] {<<expr>>}                -- Group DocumentNumber

                       Footer                                                                    Total Value               ?????

I renamed the <<expr>> into [ItemCost]. And the formula for that expression is Rate*Qty*(1-(Disc/100))

I need to find the total value, i.e the Sum of [ItemCost] for the group DocumentNumber. What is the syntax for it? [ItemCost] is not a database field and it is calculated only in the report. 

Thanks for the reply.