how to get Return Url using querystring

I am asking for login to user while moving next page and showing message for login on popup. my login is on master page..
//code on login page

if (!string.IsNullOrEmpty(Request.QueryString["ReturnUrl"]))
        {
       
            //Response.Write(retrnurl);
            string retrnurl = Request.QueryString["ReturnUrl"].ToString();
            retrnurl = "product_title.aspx";
            Response.Redirect(retrnurl,true);
 
        }