How to block user after attempting more than 5 times login

Source code

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>

Code Behined

using System;

public partial class _Default : System.Web.UI.Page

{

protected void Page_Load(object sender, EventArgs e)

{

Label1.Visible = false;

}

protected void Button1_Click(object sender, EventArgs e)

{

int c=1;

if (TextBox1.Text == "sharad" && TextBox2.Text == "gupta")

{

Response.Redirect("a.aspx");

}

else

{

Label1.Visible = true;

Label1.Text = "Wrong user or Password";

Session["counter"] = Convert.ToInt32(Session["counter"] )+ c;

if (Convert.ToInt32(Session["counter"]) > 5)

{

Label1.Text = "More Than 5 attamp User blocked!";

Button1.Visible = false;

}