Prevent direct access to page by typing URL

Hi.. I would like to prevent user to access the page by typing URL before login. I place the codes in login page load. but i can't run login page. Instead it runs the unauthorised page..
 
   if (Session["StaffID"] != null)
        {
            Response.Redirect("Admin_StaffCreate.aspx");
        }

        else
        {
            Response.Redirect("unauthorised.aspx");

        }
 
Thanks.
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