How to Show the Sum of Colum Total(Column Value) in Gridview Footer

<pre lang="xml"><asp:GridView ID="DebtorsGridView" runat="server" AutoGenerateColumns="False"
BackColor="White" BorderColor="#CCCCCC" BorderStyle="None" BorderWidth="1px"
CellPadding="3" Width="100%"
onselectedindexchanged="DebtorsGridView_SelectedIndexChanged">
<Columns> <asp:CommandField ShowSelectButton="True" />

                                                                 <asp:BoundField DataField="CusCode" HeaderText="CusCode" />
                                                                 <asp:BoundField DataField="CusName" HeaderText="CusName" />
<asp:BoundField DataField="NoofINV" HeaderText="INVNO"></asp:BoundField>
                                        <asp:BoundField DataField="Over 45" HeaderText="Over 45"
                                            DataFormatString="{0:N}" >
                                        <ItemStyle HorizontalAlign="Right" />
                                        </asp:BoundField>
                                        <asp:BoundField DataField="45 Day" HeaderText="45 Days"
                                            DataFormatString="{0:N}" >
                                        <ItemStyle HorizontalAlign="Right" />
                                        </asp:BoundField>
                                    <asp:BoundField DataField="30 Day" HeaderText="30 Days" HtmlEncode="False"
                                            DataFormatString="{0:N}">
                                        <ItemStyle HorizontalAlign="Right" />
                                        </asp:BoundField>
                                    <asp:BoundField DataField="15 Day" HeaderText="15 Days" DataFormatString="{0:N}" >
                                        <ItemStyle HorizontalAlign="Right" />
                                        </asp:BoundField>


                                </Columns>
                                <FooterStyle BackColor="White" ForeColor="#000066" />
                                <HeaderStyle BackColor="#006699" Font-Bold="True" ForeColor="White" />
                                <PagerStyle BackColor="White" ForeColor="#000066" HorizontalAlign="Left" />
                                <RowStyle ForeColor="#000066" />
                                <SelectedRowStyle BackColor="#669999" Font-Bold="True" ForeColor="White" />
                                <SortedAscendingCellStyle BackColor="#F1F1F1" />
                                <SortedAscendingHeaderStyle BackColor="#007DBB" />
                                <SortedDescendingCellStyle BackColor="#CAC9C9" />
                                <SortedDescendingHeaderStyle BackColor="#00547E" />



                            </asp:GridView>