class Program
{
    static void Main()
    {
        SpecialTimer specialTimer = SpecialTimer.Instance;

        specialTimer.TimeLeftChanged += TimeLeftChangedHandler;

        specialTimer.AddTime(10);

        Console.ReadLine();
    }

    private static void TimeLeftChangedHandler(int secondsLeft)
    {
        Console.WriteLine($"Time left: {secondsLeft} seconds");
    }

    public class SpecialTimer : System.Timers.Timer
    {
        private static SpecialTimer instance;
        private static readonly object lockObject = new object();

        public delegate void TimeLeftChangedHandler(int secondsLeft);

        public event TimeLeftChangedHandler? TimeLeftChanged;

        private SpecialTimer() : base()
        {
            Elapsed += TimerElapsed;
        }

        private void TimerElapsed(object sender, ElapsedEventArgs e)
        {
            Stop(); 
        }

        public void AddTime(int seconds)
        {
            if (Enabled && Interval > 0)
            {
                Stop(); 

                double newInterval = Interval + seconds * 1000;
                Interval = newInterval > 0 ? newInterval : 0;

                Start(); 
            }
            else
            {
                instance = new SpecialTimer();
                instance.Interval = seconds * 1000;
                instance.Start();
            }
        }

        public static SpecialTimer Instance
        {
            get
            {
                lock (lockObject)
                {
                    return instance ?? (instance = new SpecialTimer());
                }
            }
        }
    }
}

class Program
{
    static void Main()
    {
        SpecialTimer specialTimer = SpecialTimer.Instance;

        specialTimer.TimeLeftChanged += TimeLeftChangedHandler;

        specialTimer.AddTime(10);

        Console.ReadLine();
    }

    private static void TimeLeftChangedHandler(int secondsLeft)
    {
        Console.WriteLine($"Time left: {secondsLeft} seconds");
    }

    public class SpecialTimer : System.Timers.Timer
    {
        private static SpecialTimer instance;
        private static readonly object lockObject = new object();

        public delegate void TimeLeftChangedHandler(int secondsLeft);

        public event TimeLeftChangedHandler? TimeLeftChanged;

        private SpecialTimer() : base()
        {
            Elapsed += TimerElapsed;
        }

        private void TimerElapsed(object sender, ElapsedEventArgs e)
        {
            Stop(); 
        }

        public void AddTime(int seconds)
        {
            if (Enabled && Interval > 0)
            {
                Stop(); 

                double newInterval = Interval + seconds * 1000;
                Interval = newInterval > 0 ? newInterval : 0;

                Start(); 
            }
            else
            {
                instance = new SpecialTimer();
                instance.Interval = seconds * 1000;
                instance.Start();
            }
        }

        public static SpecialTimer Instance
        {
            get
            {
                lock (lockObject)
                {
                    return instance ?? (instance = new SpecialTimer());
                }
            }
        }
    }
}