static void Main(string[] args)
        {
            string[] users = {"adam"};
            start:
            printc("Enter your username:");
            string username = readline();

            for (int i = 0; i < users.Length; i++)
            {
                if (username == users[i])
                {
                    printc("Enter password:");
                    string password = readline();
                }
                else
                {
                    printc("Invalid username.");
                    goto start;
                }
            }
        }