#include <bits/stdc++.h>
using namespace std;

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int N; cin >> N;
    vector<int> B(N);
    for(auto &b : B) cin >> b;

    int answer = 0;
    for(int d=0; d<20; d++){
        bool zero = true,one = true;
        for(int i=0; i<N; i++){
            int b = B.at(i);
            int v = (b>>d)&1;
            if((i+1)&(1<<d)){
                if(v == 1) zero = false;
                else one = false;
            }
            else{
                if(v == 1) zero = false,one = false;
            }
        }
        if(zero) continue;
        else if(one) answer += 1<<d;
        else{answer = -1; break;}
    }
    cout << answer << endl;
}