using System;
using tiny = System.SByte;

class FFN
{
  static void Main()
  {
    for (tiny ffn, n = tiny.MaxValue; n > tiny.MinValue; n--)
      if ((long)(ffn = f(f(n))) != -(long)n)
        Console.WriteLine("n={0} ffn={1} ERROR", n, ffn);
  }
  
  static tiny f(tiny n)
  {
    // TODO: 在这里发挥您的聪明才智。
  }
}

        static sbyte f(sbyte n)
        {
            System.Diagnostics.StackFrame sf = new System.Diagnostics.StackFrame(1);
            if (sf.GetILOffset() > 6)
                return (sbyte)-n;
            else
                return n;
        }

------解决方案--------------------

C# code

        static tiny f(tiny n)
        {
            if (Console.Title.EndsWith(".")) {
                Console.Title = Console.Title.Substring(0, Console.Title.Length - 1);
                return (tiny)(-n);
            } else {
                Console.Title += ".";
                return n;
            }
        }