#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)

using namespace std;

int main() {
    int n;
    cin >> n;
    
    vector<int> b(n);
    rep(i, n) cin >> b[i];
    
    int m = 20;
    vector<int> ex(m, -1);
    rep(i, n) {
        rep(k, m) {
            int ib = (i+1)>>k&1;
            int bb = b[i]>>k&1;
            if (ib == 0) {
                if (bb == 1) {
                    puts("-1");
                    return 0;
                }
            }
            else {
                if (ex[k] == -1) ex[k] = bb;
                else if (ex[k] != bb) {
                    puts("-1");
                    return 0;
                }
            }
        } 
    }
    
    int ans = 0;
    rep(k, m) if (ex[k] == 1) ans |= 1<<k; 
    
    if (ans == 0) {
        int p = -1;
        rep(k, m) {
            if (ex[k] == -1) {
                p = k;
                break;
            }
        }
        if (p == -1) p = m;
        ans |= 1<<p;
    }
    
    cout << ans << '\n';
    
    return 0;
}