/* -*- coding: utf-8 -*-
 *
 * 3249.cc:  No.3249 AND - yukicoder
 */

#include<cstdio>
#include<algorithm>

using namespace std;

/* constant */

const int MAX_N = 1000000;

/* typedef */

/* global variables */

int bs[MAX_N + 1];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) scanf("%d", bs + i);

  int x = 0;
  for (int i = 1; i <= n; i++) x |= bs[i];
				 
  for (int i = 1; i <= n; i++) {
    if ((bs[i] & i) != bs[i]) { puts("-1"); return 0; }
    x &= (~i | bs[i]);
  }

  bool f = true;
  for (int i = 1; f && i <= n; i++) f = (bs[i] == (i & x));

  printf("%d\n", f ? x : -1);
  
  return 0;
}