User does not exist error

The error here is 

Error 2 The name 'User' does not exist in the current context F:\Sarah\Work\Web Development\ASP.NET.FRAME WORK\Projects\OnSet\MasterPage.master.cs 17 17 OnSet

whats the solution? please

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Web;
 using System.Web.UI;
 using System.Web.UI.WebControls;
 using System.Web.Security;

 public partial class pages_MasterPage : System.Web.UI.MasterPage
    {
     protected void Page_Load(object sender, EventArgs e)
     {
         if (!IsPostBack)
         {
             if (User.Identity.IsAuthenticated == true)
             {
                 if (Request.QueryString["username"] == "mohamed" && Request.QueryString["password"] == "1234")
                 {
                     liadmin.Visible = true;
                 }
                 // The user is logged in (you can access their username via User.Identity.Name)
             }             
         }
     }