static void Main(string[] args)
    {
      // default data
      string systemUsername = "john";
      string systemPassword = "doe";
      string username = "";
      string password = "";

      // Welcome screen
      Console.WriteLine("Welcome to our system");

      bool validCredentials = false;

      while (validCredentials == false)
      {
        Console.WriteLine("Enter username:");
        username = Console.ReadLine();
        Console.WriteLine("Enter password:");
        password = Console.ReadLine();

        if (username == systemUsername)
        {
          if (password == systemPassword)
          {
            validCredentials = true;
            Console.WriteLine("Welcome to the system!");
          }
          else
          {
            Console.WriteLine("Incorrect password!");
          }
        }
        else
        {
          Console.WriteLine("No such user found!");
        }

      }




      // system pause // press key to exit
      Console.ReadKey();
    }