using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        int[] array = { 1, 1, 4, 4, 4, 4, 8 };

        Console.WriteLine(OnlyUniqueBitwise(array));

        //Console.WriteLine("XOR Result of 5 ^ -3: " + (5 ^ -3));

    }

    public static int OnlyUnique(int[] arr) 

    {

        Dictionary<int, bool> uniques = new Dictionary<int, bool>();

        foreach (int val in arr) 

        {

            if (uniques.ContainsKey(val))

                uniques[val] = false;

            else

                uniques.Add(val, true);

        }

        IEnumerable<int> onlyUniques = uniques.Where(a => a.Value == true).Select(a => a.Key);

        return onlyUniques.Count() > 1 ? -1 : onlyUniques.First();

    }

    //ONLY WORKS IF EACH DUPLICATE APPEARS AN EVEN NUMBER OF TIMES. ELSE FAILS

    public static int OnlyUniqueBitwise(int[] arr)

    {

        int x = arr[0];

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

        {

            x = x ^ arr[i];

        }

        return arr.Contains(x) ? x : -1;

    }

}