using System;

using System.Linq;

using System.Collections.Generic;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        string s = "abba";

        int[] freq = new int[26];

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

            freq[s[i] - 'a']++;

        string ret = "";

    //  for(int i = 0; i < 26; i++){if (freq[i] != 0 && freq[i] % 2 != 0){ret += Convert.ToChar(i + 'a');}}

    //  ret = String.IsNullOrEmpty(ret) ? "Empty String" : ret;

          StringBuilder sb = new StringBuilder(s);

        for (int i = 0; i < sb.Length - 1; i++) {

            if (sb[i] == sb[i + 1]) {

                sb.Remove(i, i + 2);

                    i = -1;

            }

        }

        Console.WriteLine(sb);

        if (sb.Length == 0) {

​

           sb.Clear();

            sb.Append("Empty String");

        }

        Console.WriteLine(sb);

    }

}