#if __INCLUDE_LEVEL__ == 0

#include __BASE_FILE__

void Solve() {
  int n;
  IN(n);
  vector<int> a(n);
  IN(a);

  array<int, 20> s;
  s.fill(-1);
  for (int i : Rep(0, n)) {
    int i1 = i + 1;
    int ai = a[i];
    for (int k : Rep(0, 20)) {
      int aik = ai >> k & 1;
      int i1k = i1 >> k & 1;
      if (aik && i1k) {
        if (s[k] == 0) {
          OUT(-1);
          return;
        }
        s[k] = 1;
      } else if (aik) {
        OUT(-1);
        return;
      } else if (i1k) {
        if (s[k] == 1) {
          OUT(-1);
          return;
        }
        s[k] = 0;
      } else {
      }
    }
  }
  int ans = 0;
  for (int k : Rep(0, 20)) {
    if (s[k] == 1) {
      ans |= 1 << k;
    }
  }
  OUT(ans);
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  Solve();
}

#elif __INCLUDE_LEVEL__ == 1

#include <bits/stdc++.h>

template <class T> concept MyRange = std::ranges::range<T> && !std::convertible_to<T, std::string_view>;
template <class T> concept MyTuple = std::__is_tuple_like<T>::value && !MyRange<T>;

namespace std {

istream& operator>>(istream& is, MyRange auto&& r) {
  for (auto&& e : r) is >> e;
  return is;
}
istream& operator>>(istream& is, MyTuple auto&& t) {
  apply([&](auto&... xs) { (is >> ... >> xs); }, t);
  return is;
}

ostream& operator<<(ostream& os, MyRange auto&& r) {
  auto sep = "";
  for (auto&& e : r) os << exchange(sep, " ") << e;
  return os;
}
ostream& operator<<(ostream& os, MyTuple auto&& t) {
  auto sep = "";
  apply([&](auto&... xs) { ((os << exchange(sep, " ") << xs), ...); }, t);
  return os;
}

}  // namespace std

using namespace std;

#define Rep(...) [](int l, int r) { return views::iota(min(l, r), r); }(__VA_ARGS__)
#define IN(...) (cin >> forward_as_tuple(__VA_ARGS__))
#define OUT(...) (cout << forward_as_tuple(__VA_ARGS__) << '\n')

#endif  // __INCLUDE_LEVEL__ == 1