#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <atcoder/all>
#include <bitset>
using namespace std;
using namespace atcoder;
using ll = long long;
using mint = modint998244353;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vvvll = vector<vvll>;
using vmi = vector<mint>;
using vvmi = vector<vmi>;
using vvvmi = vector<vvmi>;
#define all(a) (a).begin(), (a).end()
#define rep2(i, m, n) for (int i = (m); i < (n); ++i)
#define rep(i, n) rep2(i, 0, n)
#define drep2(i, m, n) for (int i = (m)-1; i >= (n); --i)
#define drep(i, n) drep2(i, n, 0)


int main(){

    int n; cin >> n;
    vector<bitset<32>> vb(n);
    int mx = 0;
    rep(i, n){
        int b; cin >> b;
        mx = max(mx, b);
        bitset<32> bs(b);
        vb[i] = bs;
    }
    if(mx == 0){
        int r = 1;
        while(r <= n)r*=2;
        cout << r << endl;
        return 0;
    }
    bitset<32> ans(0);
    bool a = true;
    rep(i, 32){
        bool f = false;
        rep(j, 2){
            bool g = true;
            rep(k, n){
                bitset<32> bk(k+1);
                if((bk[i] & j) != vb[k][i]){
                    g = false;
                    break;
                }
            }
            if(g){
                ans[i] = j;
                f = true;
            }
            if(f)break;
        }
        if(!f){
            a = false;
            break;
        }
    }
    if(!a){
        cout << -1 << endl;
        return 0;
    }
    cout << ans.to_ulong() << endl;
    return 0;
}