#include <bits/stdc++.h>

#include <variant>

#define rep2(i, k, n) for (i64 i = (i64)(k); i < (i64)(n); i++)
#define rep(i, n) rep2(i, 0, n)
#define all(x) begin(x), end(x)
#ifdef ENV_LOCAL
#define dump \
  if (1) cerr
#else
#define dump \
  if (0) cerr
#endif

using namespace std;
using namespace std::string_literals;

using i32 = int32_t;
using i64 = int64_t;
using f64 = double;
using f80 = long double;
using vi32 = vector<i32>;
using vi64 = vector<i64>;

// using namespace harudake;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int n;
  cin >> n;
  i32 ans = 0;
  vi32 va(n);
  bool invalid = false;
  rep(i, n) {
    int idx = i + 1;
    i32 a;
    cin >> a;
    if ((a & idx) != a) invalid = true;
    ans |= a;
    va[i] = a;
  }
  if (ans == 0) {
    ans = 1 << (ilogb(n) + 1);
  }
  rep(i, n) {
    int idx = i + 1;
    if ((ans & idx) != va[i]) invalid = true;
  }
  if (invalid) ans = -1;
  cout << ans << "\n";
  return 0;
}