C# Back Button - Server Side (Not Javascript)

protected void Page_Load(object sender, EventArgs e)
{
 if (!IsPostBack)
 {
  ViewState["originalUrl"] = Request.UrlReferrer.AbsoluteUri;
 }
}

protected void btnBack_Click(object sender, EventArgs e)
{
 Response.Redirect(ViewState["originalUrl"].ToString(), false);
}