How to calculate two or more checked item from gridview check box in VB.NET and calculation show in textbox

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<td class="style7" colspan="2">
                        <asp:GridView ID="gvheaddetail" runat="server" AutoGenerateColumns="False" 
                            CssClass="pn" EnableModelValidation="True">
                            <Columns>
                                <asp:TemplateField HeaderText="struc" Visible="False">
                                    <ItemTemplate>
                                        <asp:Label ID="lblstruc" runat="server" Text='<%#bind("balanceid") %>'></asp:Label>
                                    </ItemTemplate>
                                </asp:TemplateField>
                                <asp:TemplateField HeaderText="hecode" Visible="False">
                                    <ItemTemplate>
                                        <asp:Label ID="lblhecode" runat="server" Text='<%#bind("Head_Code") %>'></asp:Label>
                                    </ItemTemplate>
                                </asp:TemplateField>
                                <asp:TemplateField HeaderText="Sr No." Visible="False">
                                    <ItemTemplate>
                                        <%# Container.DataItemIndex + 1 %>
                                    </ItemTemplate>
                                </asp:TemplateField>
                                <asp:TemplateField>
                                    <ItemTemplate>
                                        <asp:CheckBox ID="Checkselect" runat="server" />
                                    </ItemTemplate>
                                    <ControlStyle Width="30px" />
                                    <FooterStyle HorizontalAlign="Center" VerticalAlign="Middle" Width="30px" />
                                    <HeaderStyle HorizontalAlign="Center" VerticalAlign="Middle" Width="30px" />
                                    <ItemStyle HorizontalAlign="Center" VerticalAlign="Middle" Width="30px" />
                                </asp:TemplateField>
                                <asp:TemplateField HeaderText="Session">
                                    <ItemTemplate>
                                        <asp:Label ID="lblsession" runat="server" Text='<%#bind("Session_Name") %>'></asp:Label>
                                    </ItemTemplate>
                                    <ControlStyle Width="70px" />
                                    <FooterStyle Width="70px" />
                                    <HeaderStyle Width="70px" />
                                    <ItemStyle Width="70px" />
                                </asp:TemplateField>
                                <asp:TemplateField HeaderText="Head Name">
                                    <ItemTemplate>
                                        <asp:Label ID="lblheadname" runat="server" Text='<%#bind("HeadName") %>'></asp:Label>
                                    </ItemTemplate>
                                    <ControlStyle Width="150px" />
                                    <FooterStyle Width="150px" />
                                    <HeaderStyle Width="150px" />
                                    <ItemStyle Width="150px" />
                                </asp:TemplateField>
                                <asp:TemplateField HeaderText="Amount">
                                    <ItemTemplate>
                                        <asp:Label ID="lblamt" runat="server" Text='<%#bind("Amount") %>' onchange="calculate()"></asp:Label>
                                    </ItemTemplate>
                                    <ControlStyle Width="70px" />
                                    <FooterStyle Width="70px" />
                                    <HeaderStyle Width="70px" />
                                    <ItemStyle Width="70px" />
                                </asp:TemplateField>
                            </Columns>
                        </asp:GridView>
                    </td>

What I have tried:

Checked on gridview check box checked then Amount of row should be show in another text box who defined as lblTotal.text in this program.
All Data in gridview are pick dynamically.
Please Help me.
Thanks in advance