for循环例题

1· 一对幼兔一个月后长成小兔（每对兔子默认一公一母），再过一个月长成成兔并且生下一对小兔，以此类推，两年后有多少对兔子？

            Console.Write("输入年：");
            int a = int.Parse(Console .ReadLine ());
            int chengtu = 0;                                     //定义成兔
            int xiaotu = 0;                                      //定义小兔
            int youtu = 0;                                      //定义幼兔
            int he = 0;                                          //计算兔子的总和
            for (int i = 1; i <= 12 * a;i++ )
            {
                if (i == 1)
                {
                    chengtu = 0;
                    xiaotu = 0;
                    youtu = 1;
                }
                else
                {
                    chengtu = chengtu + xiaotu;               //这个月的成兔=上个月的成兔+上个月的小兔
                    xiaotu = youtu;                                //这个月的小兔=上个月的幼兔
                    youtu = chengtu;                             //这个月的幼兔=上个月的成兔
                    he = chengtu + xiaotu + youtu;
                }
            }
            Console.WriteLine("幼兔："+youtu+"、小兔："+xiaotu+"、成兔："+chengtu+"、总和："+he);
            Console.ReadLine();