C# 斐波那契数列 第n项数字/前n项的和

  static void Main(string[] args)
         {
             int a = Convert.ToInt32(Console.ReadLine());

             //求第n位数字是多少
             Console.WriteLine(F1(a));

             //求前n项的和
             Console.WriteLine(sum(a));
             Console.ReadKey();

         }

         /// <summary>
         /// 求第n位的数是几
         /// </summary>
         /// <param name="a"></param>
         /// <returns></returns>
         private static int F1(int a)
         {
             if (a ==  || a == )
                 return ;
             else
                 return F1(a - ) + F1(a - );
         }

         /// <summary>
         /// 求前n项的和
         /// </summary>
         /// <param name="num"></param>
         /// <returns></returns>
         private static int sum(int num)
         {
             int sum_new=;
             for (int i = ; i <= num; i++)
             {
                 sum_new =sum_new+ F1(i);
             }
             return sum_new;
         }