//
// Created by yamunaku on 2020/02/11.
//

#include <bits/stdc++.h>

using namespace std;

#define rep(i, n) for(int i = 0; i < (n); i++)
#define repl(i, l, r) for(int i = (l); i < (r); i++)
#define per(i, n) for(int i = ((n)-1); i >= 0; i--)
#define perl(i, l, r) for(int i = ((r)-1); i >= (l); i--)
#define all(x) (x).begin(),(x).end()
#define MOD9 998244353
#define MOD1 1000000007
#define IINF 1000000000
#define LINF 1000000000000000000
#define SP <<" "<<
#define CYES cout<<"Yes"<<endl
#define CNO cout<<"No"<<endl
#define CFS cin.tie(0);ios::sync_with_stdio(false)
#define CST(x) cout<<fixed<<setprecision(x)

using ll = long long;
using ld = long double;
using vi = vector<int>;
using mti = vector<vector<int>>;
using vl = vector<ll>;
using mtl = vector<vector<ll>>;
using pi = pair<int, int>;
using pl = pair<ll, ll>;
template<typename T>
using heap = priority_queue<T, vector<T>, function<bool(const T, const T)>>;

ll gcd(ll x, ll y){
    if(y == 0) return x;
    ll sz;
    while(x % y){
        sz = x % y;
        x = y;
        y = sz;
    }
    return y;
}

ll lcm(ll x, ll y){
    return x / gcd(x, y) * y;
}


unsigned long xor128() {
    static unsigned long x=123456789, y=362436069, z=521288629, w=88675123;
    unsigned long t=(x^(x<<11));
    x=y; y=z; z=w;
    return ( w=(w^(w>>19))^(t^(t>>8)) );
}


int main(){
    // CFS;
    int n;
    cin >> n;
    int lg = 0, sz = 1;
    while(n > sz) lg++, sz *= 2;
    vl a(sz, 0);
    rep(i, n){
        cin >> a[i];
        if(a[i] == -1) a[i] = 0;
    }

    rep(t, lg){
        rep(i, sz){
            if((i & (1 << t)) == 0){
                a[i] += a[i ^ (1 << t)];
            }
        }
    }
    vl b(sz), c(sz);
    const ll m = 1000000000;
    rep(i, sz){
        b[i] = a[i] / m;
        a[i] = a[i] % m;

        c[i] = b[i] * b[i];
        b[i] = 2 * a[i] * b[i];
        a[i] = a[i] * a[i];
    }
    rep(t, lg){
        rep(i, sz){
            if((i & (1 << t)) == 0){
                c[i] -= c[i ^ (1 << t)];
                b[i] -= b[i ^ (1 << t)];
                a[i] -= a[i ^ (1 << t)];
            }
        }
    }

    ll ans = (c[0] * m + b[0]) * m + a[0];
    repl(i, 1, n){
        ans = gcd(ans, (c[i] * m + b[i]) * m + a[i]);
    }
    if(ans == 0) cout << -1 << endl;
    else cout << ans << endl;
    return 0;
}