using System;
using System.Linq;
using System.Collections.Generic;

namespace Contest
{
  class Program
  {
    static void Main(string[] args)
    {
      ABC172.F();
    }
    class ABC172
    {
      public static void F()
      {
        var N = Read.Int();
        var A = Read.LongN();
        Console.WriteLine(F_My(N, A));
        //Random rand = new Random(1);
        //var A = new long[3];
        //for (int i = 0; i < 100000; ++i)
        //{
        //  for (int j = 0; j < 3; ++j)
        //  {
        //    A[j] = rand.Next() % 1000 + 1;
        //  }
        //  var ans1 = F_AC(3, A);
        //  var ans2 = F_My(3, A);
        //  if (ans1 != ans2)
        //  {
        //    Console.WriteLine("{0} {1} {2} {3} {4} {5}", ans1 == ans2, ans1, ans2, A[0], A[1], A[2]);
        //  }
        //}
        //F_My(3, new long[] { 15, 13, 20 });
      }
      static long F_My(int N, long[] A)
      {
        long res = 0;
        for (int i = 2; i < N; ++i)
        {
          res ^= A[i];
        }
        var sum = A[0] + A[1];
        long only1 = 0;
        for (int i = 0; i < 50; ++i)
        {
          if ((res & (1L << i)) > 0)
          {
            only1 += 1L << i;
          }
        }
        if (only1 > sum)
        {
          return -1;
        }
        long both = sum - only1;
        if (both % 2 > 0) { return -1; }
        var both1 = new bool[50];
        for (int i = 49; i >= 0; --i)
        {
          if ((1L << i) <= both / 2 && both / 2 < (1L << i) * 2)
          {
            both1[i] = true;
            both -= (1L << i) * 2;
          }
        }
        if (both > 0) { return -1; }
        long result1 = 0;
        for (int i = 0; i < 50; ++i)
        {
          if (both1[i]) { result1 += 1L << i; }
        }
        var SUM1 = new long[51];
        for (int i = 0; i < 50; ++i)
        {
          SUM1[i + 1] = SUM1[i] + ((res & (1L << i)) > 0 ? (1L << i) : 0);
        }
        if (result1 + SUM1[50] < A[1]) { return -1; }
        for (int i = 49; i >= 0; --i)
        {
          if (((res & (1L << i)) > 0))
          {
            var diff = result1 - A[1];
            if (diff < 0 && SUM1[i] < -diff)
            {
              result1 += 1L << i;
            }
          }
        }
        if (result1 >= sum) { return -1; }
        if (((A[0] - result1 + A[1]) ^ result1) != res) { return -1; }
        return result1 - A[1];
      }
    }
    class Util
    {
      public static T[,] Gen<T>(int r, int c, T v)
      {
        var matrix = new T[r, c];
        for (int i = 0; i < r; ++i)
        {
          for (int j = 0; j < c; ++j)
          {
            matrix[i, j] = v;
          }
        }
        return matrix;
      }
      public static long[] ASum(long[] array)
      {
        var res = new long[array.Length + 1];
        for (int i = 0; i < array.Length; ++i)
        {
          res[i + 1] = res[i] + array[i];
        }
        return res;
      }
      public static long[,] ASum(long[,] matrix)
      {
        var N = matrix.GetLength(0);
        var M = matrix.GetLength(1);
        var res = new long[N + 1, M + 1];
        for(int i = 0; i < N; ++i)
        {
          for(int j = 0; j < M; ++j)
          {
            res[i + 1, j + 1] = res[i, j + 1] + res[i + 1, j] - res[i, j] + matrix[i, j];
          }
        }
        return res;
      }
      public static Dictionary<T, long> Frequency<T>(T[] array)
      {
        var dict = new Dictionary<T, long>();
        foreach (var a in array)
        {
          if (!dict.ContainsKey(a)) { dict.Add(a, 0); }
          dict[a]++;
        }
        return dict;
      }

    }

    static class Read
{
static Queue<string> queue = new Queue<string>();
static void Enqueue() { var ss = Console.ReadLine().Split(' '); foreach (var s in ss) { queue.Enqueue(s); } }
public static int Int() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt32(queue.Dequeue()); }
public static long Long() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt64(queue.Dequeue()); }
public static string Str() { if (queue.Count == 0) { Enqueue(); } return queue.Dequeue(); }
public static int[] IntN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt32(s)).ToArray(); }
public static long[] LongN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt64(s)).ToArray(); }
public static ulong[] ULongN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToUInt64(s)).ToArray(); }
public static string[] StrN() { return Console.ReadLine().Split(' '); }
public static Tuple<int, int>[] TupleInt(int n) { List<Tuple<int, int>> list = new List<Tuple<int, int>>(); for (int i = 0; i < n; ++i) { var ab = Read.IntN(); list.Add(new Tuple<int, int>(ab[0], ab[1])); } return list.ToArray(); }
public static Tuple<long, long>[] TupleLong(int n) { List<Tuple<long, long>> list = new List<Tuple<long, long>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<long, long>(ab[0], ab[1])); } return list.ToArray(); }
public static Tuple<string, string>[] TupleStr(int n) { List<Tuple<string, string>> list = new List<Tuple<string, string>>(); for (int i = 0; i < n; ++i) { var ab = Read.StrN(); list.Add(new Tuple<string, string>(ab[0], ab[1])); } return list.ToArray(); }
public static Tuple<double, double>[] TupleDouble(int n) { List<Tuple<double, double>> list = new List<Tuple<double, double>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<double, double>(ab[0], ab[1])); } return list.ToArray(); }
public static long[,] LongMatrix(int r, int c) { var mat = new long[r, c]; for (int i = 0; i < r; ++i) { var x = Read.LongN(); for (int j = 0; j < c; ++j) { mat[i, j] = x[j]; } } return mat; }
public static Dictionary<int, HashSet<int>> AdjacencyList(int N, int M, bool isDirectional = false)
{
Dictionary<int, HashSet<int>> dict = new Dictionary<int, HashSet<int>>();
for (int i = 0; i <= N; ++i) { dict.Add(i, new HashSet<int>()); }
        for (int i = 0; i < M; ++i) { var ab = Read.IntN(); if (!dict[ab[0]].Contains(ab[1])) { dict[ab[0]].Add(ab[1]); } if (!isDirectional && !dict[ab[1]].Contains(ab[0])) { dict[ab[1]].Add(ab[0]); } }
        return dict;
}
public static Dictionary<int, Dictionary<int, long>> AdjacencyListWithWeight(int N, int M)
{
Dictionary<int, Dictionary<int, long>> dict = new Dictionary<int, Dictionary<int, long>>(); for (int i = 0; i <= N; ++i) { dict.Add(i, new Dictionary<int, long>()); }
for (int i = 0; i < M; ++i) { var ab = Read.IntN(); dict[ab[0]].Add(ab[1], ab[2]); dict[ab[1]].Add(ab[0], ab[2]); }
return dict;
}
}
    }
}


Measured memory_bytes: 19696