How to sum checked cells in GridView in footer and pass it on another page

<asp:GridView ID="GridView1" ShowFooter="true" OnRowDataBound="GridView1_RowDataBound" runat="server" AutoGenerateColumns="False" DataKeyNames="korisnikID" DataSourceID="SqlDataSource1" Height="320px" Width="763px" CellPadding="4" Font-Names="Calibri" ForeColor="#333333" HorizontalAlign="Center" font-size="Medium" GridLines="None" CssClass="auto-style9" >

<FooterTemplate>
                <asp:Label ID="Label2" runat="server" Text="Вкупно сума за плаќање"></asp:Label>
            </FooterTemplate>

                 </asp:TemplateField>

        <asp:TemplateField HeaderText="Должи" HeaderStyle-Width="100px">
<ItemTemplate>
<asp:Label ID="lblListPrice" runat="server" Text='<%# int.Parse(Eval("Должи").ToString())%>'/> </Label>
</ItemTemplate>

            <FooterTemplate>
                <asp:Label ID="lblTotal" runat="server" Text=""></asp:Label>
            </FooterTemplate>
        </asp:TemplateField>
             <asp:TemplateField HeaderText="Означи за плаќање">
                <ItemTemplate>
                    <asp:CheckBox ID="CheckBox1"  runat="server" onclick="javascript: DisableButton()"/>

                </ItemTemplate>
            </asp:TemplateField>
        </Columns>

  protected void Button2_Click(object sender, EventArgs e)
        {
            DataTable dt = new DataTable();
            dt.Columns.Add("Корисник", typeof(string));
            dt.Columns.Add("Износ", typeof(int));
            dt.Columns.Add("Побарува", typeof(int));
            dt.Columns.Add("Должи", typeof(int));

 SqlConnection sqlConn = new SqlConnection(Web.GetConfigValue("proekt1"));
            CheckBox ch;
            string fakturi = "";
            foreach (GridViewRow row in GridView1.Rows)
            {              
                int n = row.RowIndex;               
                ch = (CheckBox)row.FindControl("CheckBox1");
                if (ch.Checked == true)
                {
                    Label lblCol1 = row.FindControl("lbliznos") as Label;
                    Label lblCol2 = row.FindControl("lblpobaruva") as Label;
                    Label lblCol3 = row.FindControl("lblListPrice") as Label;
                    if (lblCol1 != null && lblCol2 != null && lblCol3 != null)
                    {
                        int val1 = int.Parse(lblCol1.Text);
                        int val2 = int.Parse(lblCol2.Text);
                        int val3 = int.Parse(lblCol3.Text);
                        lblCol2.Text = (val2 + val3).ToString();
                        lblCol3.Text = (val3 * 0).ToString();
                    }
                    DataRow r = dt.NewRow();
                    r["Корисник"] = Web.SessionUser.Naziv.ToString(); //row.Cells[1].Text;
                    Label name2 = (Label)GridView1.Rows[n].Cells[3].FindControl("lbliznos");
                    r["Износ"] = name2.Text;
                    Label name = (Label)GridView1.Rows[n].Cells[5].FindControl("lblpobaruva");
                    r["Побарува"] = name.Text;
                    Label name1 = (Label)GridView1.Rows[n].Cells[6].FindControl("lblListPrice");
                    r["Должи"] = name1.Text;
                    if (fakturi != "") fakturi += ",";
                    fakturi += row.Cells[0].Text;
                    dt.Rows.Add(r);
                }
            }
            Session["platiltabela"] = dt;
            Session["fakturi"] = fakturi;
            Response.Redirect("Naplata.aspx");
        }

  protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
        {
            if (e.Row.RowType == DataControlRowType.DataRow)
            {
                CheckBox chk = (CheckBox)e.Row.FindControl("CheckBox1");
                Label name1 = (Label)e.Row.FindControl("lblListPrice");
                if (name1.Text == "0")
                {
                    chk.Enabled = false;
                }
                else
                {
                    chk.Enabled = true;
                }
                if (chk.Checked == false)
                {
                    Button2.Enabled = false;
                }
                else { Button2.Enabled = true; }
            }
        }
    }
}