How to make sum of 2 columns in grid and display the result in third column?

<asp:GridView runat="server" ID="GV" OnRowDataBound="test_rodatbond">
    <Columns>
    <asp:BoundField DataField="v1" />
    <asp:BoundField DataField="v2" />
    <asp:TemplateField>
    <ItemTemplate>
    <asp:Label ID="lbl" runat="server"></asp:Label>
    </ItemTemplate>
    </asp:TemplateField>
    </Columns>
    </asp:GridView>

protected void test_rodatbond(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            Label lbl = (Label)e.Row.FindControl("lbl");
            lbl.Text = GV.Rows[e.Row.RowIndex].Cells[0].Text + GV.Rows[e.Row.RowIndex].Cells[1].Text;

        }
    }