#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, t) for (ll i = s; i < (ll)(t); i++)
#define all(x) begin(x), end(x)

template <class T> bool chmin(T& x, T y) {
	return x > y ? (x = y, true) : false;
}
template <class T> bool chmax(T& x, T y) {
	return x < y ? (x = y, true) : false;
}

struct io_setup {
	io_setup() {
		ios::sync_with_stdio(false);
		cin.tie(nullptr);
		cout << fixed << setprecision(15);
	}
} io_setup;

ll lpow(ll a, ll x) {
	ll res = 1;
	rep(i, 0, x) res *= a;
	return res;
}

void solve() {
	int n;
	cin >> n;
	vector<int> b(n);
	rep(i, 0, n) cin >> b[i];
	ll ans = 0;
	rep(i, 0, n) {
		ans |= (i + 1) & b[i];
	}
	rep(i, 0, n) {
		if (((i + 1) & ans) != b[i]) {
			cout << "-1\n";
			return;
		}
	}
	cout << ans << "\n";
}

int main() {
	int t = 1;
	// cin >> t;
	while (t--) solve();
}