using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string einNumber ="8223690883";//82-2369088

        string test = FormatEIN(einNumber);

        Console.WriteLine(test);

    }

    public static string FormatEIN(string einNumber)

    {

        string returnVal = "Valid";

        Regex regex = new Regex(@"^[1-9]\d?-\d{7}$");

        var isValid =   regex.IsMatch(einNumber);

        if(isValid)

        {

            returnVal =  einNumber;

        }

        else if(einNumber.Length == 9 && einNumber.Contains("-") == false)

        {

            returnVal = Regex.Replace(einNumber,@"^(.{2})(.{7})","$1-$2");

        }

        else{

            returnVal =  "Not Valid";

        }

        return returnVal;

    }

}