锁线程的问题，为啥计算结果不对

[Rainlonely]

        public const long max = 100;
        static string buffer = "0";
        private object hoho = new object();
        static void Main(string[] args)
        {
           
            Program test = new Program();
            Thread a = new Thread(new ThreadStart(test.aDowork));
            a.Start();
            Thread b = new Thread(new ThreadStart(test.bDowork));
            b.Start();
            Thread c = new Thread(new ThreadStart(test.cDowork));
            c.Start();
            Thread d = new Thread(new ThreadStart(test.dDowork));
            d.Start();
            a.Join();
            b.Join();
            c.Join();
            d.Join();
            Console.WriteLine("Resultis{0}", buffer);
        }

        void aDowork()
        {
            for (long i = 1; i < max / 4; i++)
            {
               lock(hoho)
               {
                   buffer = (long.Parse(buffer) + i).ToString() ;
                   Console.WriteLine("{0}+{1}", buffer, i);
                   Console.ReadKey();
               }
            }
        }

        void bDowork()
        {
            for (long i = max / 4; i < max/2; i++)
            {
                lock (hoho) { buffer = (long.Parse(buffer) + i).ToString(); }
                Console.WriteLine("{0}+{1}", buffer, i);

            }
        }

        void cDowork()
        {
            for (long i = max / 2; i < max * 3 / 4; i++)
            {
                lock (hoho) { buffer = (long.Parse(buffer)).ToString(); }
                Console.WriteLine("{0}+{1}", buffer, i);

            }
        }
        void dDowork()
        {
            for(long i = max*3/4;i<=max;i++)
            {
                lock (hoho) { buffer = (long.Parse(buffer)).ToString(); }
                Console.WriteLine("{0}+{1}", buffer, i);

            }
        }

一直以为这个线程是执行完一个 然后进行下一个 再计算， 调试的时候发现时abcdabcd这么来的 可是 最后的结果还是不对的， 哪位高手能解释一下？？

[Rainlonely]

我错了 c和d 忘记+i了