Renaming same image with counter

if (FileUpload1.HasFile)
            {
                string path = Server.MapPath("~/img/");
                string filename = FileUpload1.FileName;
                string ext = Path.GetExtension(filename);
                string avilablefile = filename;
                if (ext == ".jpg" || ext == ".jpeg" || ext == ".gif" || ext == ".png")
                {
                    string concate = path + filename;
                    int counter = 2;
                    while (System.IO.File.Exists(concate))
                    {
                        avilablefile = counter.ToString() + filename;
                        concate = path + avilablefile;
                        counter++;
                    }
                    FileUpload1.SaveAs(concate);
               }
         }