using System;

public class Program

{

    public static void Main()

    {

        try{

            Console.WriteLine(GetTransactionNumerFormatted(-30));

        }

        catch(InvalidOperationException ex){

            Console.WriteLine(ex);

        }

        Console.WriteLine(GetTransactionNumerFormatted(1));

        Console.WriteLine(GetTransactionNumerFormatted(12));

        Console.WriteLine(GetTransactionNumerFormatted(123));

        Console.WriteLine(GetTransactionNumerFormatted(1234));

        Console.WriteLine(GetTransactionNumerFormatted(12345));

    }

    private static string GetTransactionNumerFormatted(int transactionId){

        if (transactionId < 0) {

            throw new InvalidOperationException("Transaction ID should be possitive");

        }

        var transactionStr = transactionId.ToString();

        if (transactionId < 10000){

            return transactionStr.PadLeft(4, '0');

        }

        return transactionStr.Substring(transactionStr.Length -4);

    }

}