Display nested ASP.Net GridView column total (sum) in Bootstrap modal popup

Hi,

how can I total up lblOrderAmount after clicking the save button in [GridView2]?

My goal is to update lblTotalAmount accordingly in [GridView1] TQ HTML

<div>
    <asp:GridView ID="GridView1" runat="server" CssClass="table table-sorting table-hover table-bordered datatable" DataKeyNames="student_Id" AutoGenerateColumns="false" GridLines="None">
        <Columns>
            <asp:TemplateField HeaderText="Admission No">
                <ItemTemplate>
                    <asp:GridView ID="GridView2" runat="server" CssClass="table table-sorting table-hover table-bordered datatable" AutoGenerateColumns="false" GridLines="None">
                        <Columns>
                            <asp:TemplateField HeaderText="Order Amount">
                                <ItemTemplate>
                                    <asp:Label ID="lblOrderAmount" runat="server" Text='<%# Eval("OrderAmount") %>'></asp:Label>
                                </ItemTemplate>
                            </asp:TemplateField>
                            <asp:TemplateField HeaderText="Pay">
                                <ItemTemplate>
                                    <asp:LinkButton ID="lnkBtnEdit" runat="server" Text="Update" CssClass="btn btn-info" OnClick="Display"></asp:LinkButton>
                                </ItemTemplate>
                            </asp:TemplateField>
                        </Columns>
                    </asp:GridView>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Admission No">
                <ItemTemplate>
                    <asp:Label ID="lblstudent_Id" runat="server" Text='<%# Eval("student_Id") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Total Amount">
                <ItemTemplate>
                    <asp:Label ID="lblTotalAmount" runat="server" Text='<%# Eval("TotalAmount") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
</div>
<div>
    <div class="modal fade" id="myModal" role="dialog">
        <div class="modal-dialog">
            <!-- Modal content-->
            <div class="modal-content">
                <div class="modal-header">
                    <button type="button" class="close" data-dismiss="modal">&times;</button>
                    <h4 class="modal-title">Student Fee Details</h4>
                </div>
                <div class="modal-body">
                    <div class="col-lg-12 col-sm-12 col-md-12 col-xs-12">
                        <div class="form-group">
                            <asp:Label ID="lblstudent" runat="server" Text="Admission No: "></asp:Label>
                            <asp:Label ID="lblstudentid" runat="server" Text=""></asp:Label>
                        </div>
                        <div class="form-group">
                            <asp:Label ID="lblAmount" runat="server" Text="Order Amount"></asp:Label>
                            <asp:TextBox ID="txtAmount" runat="server" CssClass="form-control"></asp:TextBox>
                        </div>
                    </div>
                </div>
                <div class="modal-footer">
                    <asp:Button ID="btnSave" runat="server" Text="Update" OnClick="btnSave_Click" CssClass="btn btn-info" />
                    <button type="button" class="btn btn-info" data-dismiss="modal">Close</button>
                </div>
            </div>
        </div>
        <script type='text/javascript'>
            function openModal() {
                $('[id*=myModal]').modal('show');
            }
        </script>
    </div>
</div>

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load 
    If Not IsPostBack Then 
        Dim dt As New DataTable()
        dt.Columns.AddRange(New DataColumn() {New DataColumn("student_Id"), New DataColumn("TotalAmount")}) 
        dt.Rows.Add(101, 5200)
        dt.Rows.Add(102, 6500)
        GridView1.DataSource = dt
        GridView1.DataBind() 
    End If
 
End Sub
 
Protected Sub Display(ByVal sender As Object, ByVal e As EventArgs)
    Dim rowOrder As GridViewRow = TryCast((TryCast(sender, LinkButton)).NamingContainer, GridViewRow)
    Me.txtAmount.Text = TryCast(rowOrder.FindControl("lblOrderAmount"), Label).Text 
    ClientScript.RegisterStartupScript(Me.[GetType](), "Pop", "openModal();", True) 
End Sub
 
Protected Sub btnSave_Click(ByVal sender As Object, ByVal e As EventArgs) 
    'total up of lblOrderAmount from GridView2 and update accordingly in GridView1 
End Sub
 
Protected Sub GridView1_RowDataBound(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewRowEventArgs) Handles GridView1.RowDataBound 
    If e.Row.RowType = DataControlRowType.DataRow Then 
        Dim GridView2 As GridView = TryCast(e.Row.FindControl("GridView2"), GridView) 
        Dim dt As New DataTable()
        dt.Columns.AddRange(New DataColumn() {New DataColumn("OrderAmount")})
        dt.Rows.Add(RandomInteger(1000, 9999))
        dt.Rows.Add(RandomInteger(1000, 9999))
        dt.Rows.Add(RandomInteger(1000, 9999))
 
        GridView2.DataSource = dt
        GridView2.DataBind() 
    End If 
End Sub
 
Public Function RandomInteger(ByVal LowerBoundary As Integer, ByVal UpperBoundary As Integer) As Integer 
    Randomize()    'Initialize the random-number generator
    RandomInteger = (Rnd() * (UpperBoundary - LowerBoundary)) + LowerBoundary 
End Function