seesion code doubt

Question

Default.aspx code


using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class _Default : System.Web.UI.Page
{
    string url;
    protected void Page_Load(object sender, EventArgs e)
    {
      
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        Session["a"] = txt_username.Text.ToString();
        Session["b"] = txt_password.Text.ToString();
        Response.Redirect("screen.aspx");

    }
}


screen.aspx code

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        Label1.Text = Session["a"].ToString();
        Label1.Text = Session["b"].ToString();
      

    }
}


Design page

Default.aspx design page


Session.aspx design page


when i run default.aspx and click the button
the following error occurs
the resource cannot be found.

please help me. i want to retreive the  username and password of default.aspx  to session.aspx page using  session please help me.

and send the correct code.what is wrong in my above code.