using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(Process("a1a2a3a4a5a6"));

    }       public static string Process(string input)

        {

            string output = string.Empty;

            char[] inputCharArray = input.ToCharArray();

            int[] count = new int[255];

            int i = 0;

            foreach (char c in inputCharArray)

            {

                count[(int)c]++;

                if(count[(int)c]%2 != 0) output += c;

                i++;

            }

​

            return output;

        }

}