using System;

public class Program

{

    public static void Main()

    {

        while(true) {

            Console.WriteLine("표시할 숫자를 입력하세요(종료:-1): ");

            string s = Console.ReadLine();

            double v = double.Parse(s);

​

            if (v == -1) {

                break;

            }

            Console.WriteLine(NumberWithGroupSeparator(s));

        }

    }

    private static string NumberWithGroupSeparator(string s) {

        int pos = 0;

        double v = Double.Parse(s);

​

        if (s.Contains(".")) {

            pos = s.Length - s.IndexOf('.');

            string formatStr = "{0:N" + (pos - 1) + "}";

            s = string.Format(formatStr, v);

        }

        else {

            s = string.Format("{0:N0}", v);

        }

​

        return s;

    }

}