using System;

using static System.Console;                

public class Program

{

    public static void Main()

    {

        //WriteLine(Reversed("hello"));

        WriteLine(FirstRecurringCharacter("ABCDEFGAHB"));

    }

    public static char? FirstRecurringCharacter(string input)

    {

       for(int i = 0; i<input.Length; i++)

       {

          for(int j = i + 1; j < input.Length; j++)

          {

              if(input[i] == input[j])

                 return input[i];

          }

       }

        return null;

    }

    /*

    public static string Reversed(string input)

    {

       char[] charArray = input.ToCharArray();

       Array.Reverse(charArray);

        string reversed = new string(charArray);

        return reversed;

    }

    */

}