how to show total of a column of a grid view in footer below that cell

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ASP.NET

C#5.0

I have a grid view to display the billing information. The content of gridview includes

s.no. particulars quantity perunitprice amount
1 xyz 10 1000 10000
2 abc 5 50 250
Total: 1250

here, amount is calculated by multiplying quantity with perunit price which i have done in RowDataBound of gridview.
I wanted to display total amount in footer of gridview as shown above. In order to do so, i have used template field.
Here is the client side code.

<Columns>
    <asp:TemplateField HeaderText="S. No">
        <ItemTemplate>
        <%# Container.DataItemIndex + 1 %>
        </ItemTemplate>
        </asp:TemplateField>
        <asp:BoundField DataField="particulars" HeaderText="particulars" />
                            <asp:BoundField DataField="Quantity" HeaderText="Quantity" />
                            <asp:BoundField DataField="Per_Unit_Price" HeaderText="Per Unit Price" />
                            <asp:BoundField DataField="Amount" HeaderText="Amount" />
                            <asp:TemplateField>
                                <FooterTemplate>
                                    <asp:Label ID="lblTotal" runat="server"></asp:Label>
                                </FooterTemplate>
                            </asp:TemplateField>
                        </Columns>

Although i am getting total in footer but the footer is displayed in 6th column but i wanted it as a footer of 5th column.
How can i do it.. any help???