#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n) - 1; i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r) - 1;i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	int n; cin >> n;
	vector<int>b(n);
	rep(i, n)cin >> b[i];
	vector<int>bit(30, -1);
	rep(i, n) {
		rep(j, 30) {
			int y = 1 & ((i + 1) >> j);
			int x = 1 & (b[i] >> j);
			if (x + y == 2) {
				if (bit[j] == -1) {
					bit[j] = 1;
				}
				else if (bit[j] == 0) {
					cout << -1 << endl;
					return 0;
				}
			}
			else if (x == 1 && y == 0) {
				cout << -1 << endl;
				return 0;
			}
			else if (y == 0 && x == 1) {
				if (bit[j] == -1) {
					bit[j] = 0;
				}
				else if (bit[j] == 1) {
					cout << -1 << endl;
					return 0;
				}
			}
		}
	}
	int ans = 0;
	rep(i, 30)if (bit[i] == 1)ans += 1 << i;
	cout << ans << endl;
	return 0;
}