Export GridView with Calculation in Footer to Excel in ASP.Net

HTML

 <asp:GridView ID="GridView1" HeaderStyle-BackColor="#3AC0F2" HeaderStyle-ForeColor="White"
            runat="server" AutoGenerateColumns="false" ShowFooter="true" OnDataBound="GridView1_DataBound">
            <Columns>
                <asp:BoundField DataField="Id" HeaderText="Id" ItemStyle-Width="30" />
                <asp:BoundField DataField="Name" HeaderText="Name" ItemStyle-Width="150" />
                <asp:TemplateField HeaderText="Salary">
                    <ItemTemplate>
                        <asp:Label ID="lblSalary" runat="server" Text='<%# Eval("Salary") %>' />
                    </ItemTemplate>
                    <FooterTemplate>
                        <asp:Label ID="lblTotalSalary" runat="server" />
                    </FooterTemplate>
                </asp:TemplateField>
            </Columns>
        </asp:GridView>
        <asp:Button ID="btnExport" Text="Export" OnClick="ExportExcel" runat="server" />

Code

protected void Page_Load(object sender, EventArgs e)
    {
        if (!this.IsPostBack)
        {
            DataTable dt = new DataTable();
            dt.Columns.AddRange(new DataColumn[3] { new DataColumn("Id", typeof(string)),
                            new DataColumn("Name", typeof(string)),
                            new DataColumn("Salary",typeof(int)) });
            dt.Rows.Add(1, "John Doe", 20000);
            dt.Rows.Add(2, "Suzanne Mathews", 45000);
            dt.Rows.Add(3, "Robert Schidner", 50000);
            ViewState["DataTable"] = dt;
            GridView1.DataSource = dt;
            GridView1.DataBind();
        }
    }

    protected void GridView1_DataBound(object sender, EventArgs e)
    {
        int total = (from row in GridView1.Rows.Cast<GridViewRow>()
                     select int.Parse((row.FindControl("lblSalary") as Label).Text)).Sum();
        (GridView1.FooterRow.FindControl("lblTotalSalary") as Label).Text = "Total: " + total.ToString();
        DataTable dt = (DataTable)ViewState["DataTable"];
        dt.Rows.Add("", "Total", total);
        ViewState["DataTable"] = dt;
    }

    protected void ExportExcel(object sender, EventArgs e)
    {
        DataTable dt = new DataTable("GridView_Data");
        dt = (DataTable)ViewState["DataTable"];

        using (XLWorkbook wb = new XLWorkbook())
        {
            wb.Worksheets.Add(dt);
            Response.Clear();
            Response.Buffer = true;
            Response.Charset = "";
            Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
            Response.AddHeader("content-disposition", "attachment;filename=GridView.xlsx");
            using (MemoryStream MyMemoryStream = new MemoryStream())
            {
                wb.SaveAs(MyMemoryStream);
                MyMemoryStream.WriteTo(Response.OutputStream);
                Response.Flush();
                Response.End();
            }
        }
    }

Screenshot