一只羊的四个月可以生一只小羊，小羊四个月后又可以生一只小羊 问50个月后有多少只羊（羊不会死）

        /// <summary>
        ///  获得羊
        /// </summary>
        /// <param name="day">月数</param>
        /// <param name="initialSheepNum">初始有多少只羊</param>
        /// <returns></returns>
        public ulong Sheep(int day, ulong initialSheepNum = 1)
        {
            ulong all = initialSheepNum;
            ulong a = 0;
            ulong b = 0;
            ulong c = 0;
      
            for (int i = 0; i < day; i++)
            {

                //                Console.WriteLine(all);
                all += c;
                c = b;
                b = a;
                a = all;

            }
            return all;
        }