using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        int[] A = {9, 3, 9, 3, 9, 7, 9};

        Console.WriteLine(solution(A));

    }

​

    public static int solution(int[] A)

    {

        var has = new HashSet<int>();

        foreach (var i in A)

            if (has.Contains(i))

                has.Remove(i);

            else

                has.Add(i);

        foreach (var i in has)

            return i;

        return 0;

    }

}