using System;

public class Program

{

    public static void Main()

    {

        string text = "1235612537123";

        var newText = "";

        for(int i= 0; i< text.Length; i++) {

            newText += text[i];

            if((i+1) % 2 == 0)

                newText +=",";

        }

        Console.WriteLine(newText);

    }

}