using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    static int[] NMi => ReadLine().Split().Select(c => int.Parse(c) - 1).ToArray();
    static int[][] NMap(int n) => Enumerable.Repeat(0, n).Select(_ => NMi).ToArray();
    static string[] SList(long n) => Enumerable.Repeat(0, (int)n).Select(_ => ReadLine()).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var b = NList;
        var ans = 0;
        for (var i = 0; i < n; ++i)
        {
            var tmp = i + 1;
            for (var j = 0; j < 20; ++j)
            {
                var tj = (tmp >> j) & 1;
                var bj = (b[i] >> j) & 1;
                if (tj == 0)
                {
                    if (bj == 1)
                    {
                        WriteLine(-1);
                        return;
                    }
                }
                else
                {
                    if (bj == 1) ans |= 1 << j;
                }
            }
        }
        WriteLine(ans);
    }
}