Mastahh

        static void Main(string[] args)
        {
            int size = 20;
            String s1 = "";
            for (int i = 1; i <= size; i++)
            {
                s1 = string.Format("{0}-{1}-{0}", i,s1);
                Console.WriteLine(((i % 2 == 0) ? s1.Substring(0, s1.Length / 2 + 1) : s1.Substring(s1.Length / 2)).Trim('-'));
            }
        }