działania matematyczne na wartosciach w item 1 i item 2

cześć
Mam do Was pytanie jak mógłbym zsumować

GridGroupSummaryItem item1 = new GridGroupSummaryItem();
            item1.FieldName = "a";
            item1.SummaryType = DevExpress.Data.SummaryItemType.Count;
            item1.DisplayFormat = "{0}";
            item1.ShowInGroupColumnFooter = gridView1.Columns["a"];
            gridView1.GroupSummary.Add(item1);


            GridGroupSummaryItem item2 = new GridGroupSummaryItem();
            item2.FieldName = "b";
            item2.SummaryType = DevExpress.Data.SummaryItemType.Sum;
            item2.DisplayFormat = "{0}";
            item2.ShowInGroupColumnFooter = gridView1.Columns["b"];
            gridView1.GroupSummary.Add(item2);

 GridGroupSummaryItem item3 = new GridGroupSummaryItem();
            item3.FieldName = "c";
            item3.SummaryType = DevExpress.Data.SummaryItemType.Sum;
            item3.DisplayFormat = "{0}";
            item3.ShowInGroupColumnFooter = gridView1.Columns["c"];
            gridView1.GroupSummary.Add(item3);
item3 = item1 + item2