using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        string numero = "12,126456";

        decimal d;

        if (decimal.TryParse(numero,NumberStyles.AllowDecimalPoint, CultureInfo.CreateSpecificCulture("pt-BR"), out d))

        {

           Console.WriteLine(d.ToString("N2"));

        }

        else

        {

            Console.WriteLine("Número inválido");

        }

    }

}