visitors count (online users)

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ASP.NET

I wrote the below codes for visitors count (online users) for my application (in Global.asax)
I brows my website in some browser (IE and mozilla and chrome)
the number of users increase correctly but when I close some browsers it not decrease.

void Application_Start(object sender, EventArgs e)
  {
      Application["count"] = 0;
  }
 

  void Session_Start(object sender, EventArgs e)
  {
      Application["count"] = Convert.ToInt32(Application["count"]) + 1;
  }

  void Session_End(object sender, EventArgs e)
  {    
      Application["count"] = Convert.ToInt32(Application["count"]) - 1;
  }