How to get Previous Page URL in asp.net

this is html code

<asp:Button runat="server" ID="btnBack" Text="Back" onclick="btnBack_Click" />

this is c# code

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack) //check if the webpage is loaded for the first time.
        {
            ViewState["PreviousPage"] = 
  Request.UrlReferrer;//Saves the Previous page url in ViewState
        }
    }
    protected void btnBack_Click(object sender, EventArgs e)
    {
        if (ViewState["PreviousPage"] != null) //Check if the ViewState 
      //contains Previous page URL
        {
            Response.Redirect(ViewState["PreviousPage"].ToString());//Redirect to 
  //Previous page by retrieving the PreviousPage Url from ViewState.
        }
    }