using System;

using org.matheval;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string[] s = new string[]{"1000","1,000","1,123.00","100,123.50","100.123.50","100.123.500","100.123.500,00","50","100,123,500,00"};

​

        foreach(var _s in s)

        {       

            List<string> fullsplit = new List<string>();

            var periodSplit = _s.Split('.');

            foreach(var x in periodSplit)

            {

                foreach(var y in x.Split(','))

                {

                    fullsplit.Add(y);

                }   

            }

            string result = "";

            if(fullsplit.Count > 1 && fullsplit.Last().Length < 3)

            {

                result = string.Format("{0}.{1}",string.Join("",fullsplit.ToArray(),0, fullsplit.Count - 1),fullsplit.Last());

            }

            if(fullsplit.Count == 1 || fullsplit.Last().Length >=3)

            {

                result = string.Join("",fullsplit.ToArray());

            }

            Console.WriteLine(_s+ ":  "+ result);

​

        }

    }

}