using System;

using System.Text;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(GenerateAuthCodeBad());

        Console.WriteLine(GenerateAuthCodeBad());

        Console.WriteLine(GenerateAuthCodeBad(7));

        Console.WriteLine(GenerateAuthCodeBad(2));

        Console.WriteLine(GenerateAuthCodeBad(10));

        Console.WriteLine("---------------------");

        Console.WriteLine(GenerateAuthCodeGood());

        Console.WriteLine(GenerateAuthCodeGood());

        Console.WriteLine(GenerateAuthCodeGood(7));

        Console.WriteLine(GenerateAuthCodeGood(2));

        Console.WriteLine(GenerateAuthCodeGood(10));

    }

      private static string GenerateAuthCodeBad(int numDigits = 6)

      {

          StringBuilder sb = new StringBuilder();

          Random rng = new Random();

          for (int i = 1; i <= numDigits; i++)

          {

              sb.Append(rng.Next(0, 9).ToString());

          }

          return sb.ToString();

      }

        private static Random staticRng = new Random();

      private static string GenerateAuthCodeGood(int numDigits = 6)

      {

          StringBuilder sb = new StringBuilder();         

          for (int i = 1; i <= numDigits; i++)

          {

              sb.Append(staticRng.Next(0, 9).ToString());

          }

          return sb.ToString();

      }

}