Mastahh

static void Main(string[] args)
        {
            int size = 10;
            String s1 = "" ,s2 = "";
            for (int i = 1; i <= size; i++)
            {
                s1 += i;
                s2 = i + "-" + s2.Trim('-');
                Console.WriteLine((i % 2 == 0)?s2:s1);
                s1 += "-";
            }
        }