Please refer below Code
HTML
<div>
<asp:Label ID="Lblorder" Text="2" runat="server" />
<asp:Label ID="LblTotal" Text="5" runat="server" />
<asp:Label ID="Lblsum" runat="server" />
</div>
C#
Lblsum.Text = (Convert.ToInt32(Lblorder.Text) + Convert.ToInt32(LblTotal.Text)).ToString();
I hope this will help you out.