using System;

public class Program

{

    public static void Main()

    {

        var val = 10000823833;

        var reversedValue = Helper.ReverseValue(val.ToString());

        var reversedResult = string.Empty;

        var len = reversedValue.Length - 1;

        for(int i = 0; i <= len;  i++)

        {

            reversedResult += reversedValue[i];

            var deelbaarDoor3 = ((i + 1) % 3) == 0;

            if(deelbaarDoor3 && i != len)

            {

                reversedResult += ".";

            } 

        }

        var result = Helper.ReverseValue(reversedResult);

        Console.WriteLine(result);

    }

}

​

public static class Helper

{

    public static string ReverseValue(string val)

    {

        var valArray = val.ToCharArray();

        Array.Reverse(valArray);

        return new string(valArray);

    }

}