SomeDich

static void Main(string[] args)
        {
            string s = "";
            while (s != "hare")
            {
                Console.Write("Input:");
                s = Console.ReadLine();
                long p = Convert.ToInt64(s);
                s = SizeFormat(p);
                Console.WriteLine(s);
                Console.ReadLine();
            }
        }

        static string SizeFormat(long size)
        {
            string sizeStr = size.ToString();
            string result = "";

            do
            {
                if (sizeStr.Length >= 3)
                {
                    result = sizeStr.Substring(sizeStr.Length - 3) + " " + result;
                    sizeStr = sizeStr.Remove(sizeStr.Length - 3);
                }
                else
                {
                    result = sizeStr.Substring(sizeStr.Length - 1) + " " + result;
                    sizeStr = sizeStr.Remove(sizeStr.Length - 1);
                }

            }
            while (sizeStr != "");

            if (result.StartsWith(" "))
            {
                result = result.Substring(1);
            }
            if (result.EndsWith(" "))
            {
                result = result.Substring(0, result.Length - 1);
            }

            return result;

        }