多线程程序的临界区

代码        static int i = 0; //初始面包为0
        static void Main(string[] args)
        {
            Thread t1 = new Thread(new ThreadStart(Fun));
            Thread t2 = new Thread(new ThreadStart(Fun));
            t1.Start();
            t2.Start();
        }
        static void Fun()
        {
            Thread.Sleep(100);
            while (i <= 3)
            {
                i++;
                Console.WriteLine("面包个数:{0},购买人:{1}", i, System.Threading.Thread.CurrentThread.ManagedThreadId);
            }
        }