#include <bits/stdc++.h>
using namespace std;
// #include <atcoder/modint>
// using namespace atcoder;
// using mint = modint998244353;
using ll = long long;
#define fix(x) fixed << setprecision(x)
#define rep(i, n) for(int i = 0; i < n; ++i)
#define all(x) (x).begin(),(x).end()
template<class T>bool chmin(T&a, const T&b){if(a>b){a=b;return 1;}return 0;}
template<class T>bool chmax(T&a, const T&b){if(a<b){a=b;return 1;}return 0;}
constexpr ll INFLL = (1LL << 62), MOD = 998244353;
constexpr int INF = (1 << 30);

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n;
    cin >> n;
    int k0 = 0, k1 = 0;
    for(int i=1;i<=n;++i){
        int b;
        cin >> b;
        if((i&b)!=b){
            cout << "-1\n";
            return 0;
        }
        k1 |= b;
        b ^= i;
        k0 |= b;
    }
    if(k0&k1) cout << "-1\n";
    else cout << k1 << '\n';
    return 0;
}