using System;

using System.Security.Cryptography;

using System.Text;

public class Program

{

    public static void Main()

    {

        // Fill in these values

        var originalInvoiceNr = "3";

        var originalInvoiceDate = new DateTime(2023, 01, 02);

        // Fill in these values

​

        string invoiceNr = originalInvoiceNr;

        if (originalInvoiceNr.Length > 8)

        {

            invoiceNr = originalInvoiceNr.Substring(invoiceNr.Length - 8);

        }

        else if (originalInvoiceNr.Length < 8)

        {

            for (int i = originalInvoiceNr.Length; i <= 8; i++)

            {

                invoiceNr = "0" + invoiceNr;

            }

        }

​

        var modulo = long.Parse(originalInvoiceDate.ToString("yy") + invoiceNr.ToString());

        modulo = modulo % 97;

        if (modulo == 0)

        {

            modulo = 97;

        }

        string moduloString = modulo.ToString("00");

        var structuredReference = "+++" + originalInvoiceDate.ToString("yy") + invoiceNr[0] + "/" + invoiceNr.Substring(1, 4) + "/" + invoiceNr.Substring(5, 3) + moduloString + "+++";

        Console.WriteLine(structuredReference);

    }

}