#include<bits/stdc++.h>
//ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef unsigned int ui;
typedef pair<int,int> pii;
typedef pair<pii,int> ppii;
typedef pair<int,pii> pipi;
typedef pair<ll,ll> pll;
typedef pair<pll,ll> ppll;
typedef pair<ll,pll> plpl;
typedef tuple<ll,ll,ll> tl;
ll mod=1000000007;
ll mod2=998244353;
ll mod3=1000003;
ll mod4=998244853;
ll inf=1LL<<60;
long double pi=3.14159265358979323846L;
double eps=1e-12;
#define rep(i,m,n) for(ll i=m;i<n;i++)
#define rrep(i,n,m) for(ll i=n;i>=m;i--)
#define srep(itr,st) for(auto itr=st.begin();itr!=st.end();itr++)
int dh[4]={1,-1,0,0};
int dw[4]={0,0,1,-1};
int ddh[8]={-1,-1,-1,0,0,1,1,1};
int ddw[8]={-1,0,1,-1,1,-1,0,1};
ll gcd(ll a,ll b){
    if(a<b)swap(a,b);
    if(b==0)return a;
    if(a%b==0)return b;
    return gcd(b,a%b);
}
ll Pow(ll n,ll k){
    ll ret=1;
    ll now=n;
    while(k>0){
        if(k&1)ret*=now;
        now*=now;
        k/=2;
    }
    return ret;
}
ll beki(ll n,ll k,ll md){
  ll ret=1;
  ll now=n;
  while(k>0){
    if(k%2==1){
      ret*=now;
      ret%=md;
    }
    now*=now;
    now%=md;
    k/=2;
  }
  return ret;
}
ll gyaku(ll n,ll md){
  return beki(n,md-2,md);
}random_device rnd;
mt19937 mt(rnd());

int main(){
    ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    int n;cin>>n;
    ll a[n];
    rep(i,0,n)cin>>a[i];
    bool ok[n+1];
    fill(ok,ok+n+1,true);
    ll sum[n+1];
    fill(sum,sum+n+1,0);
    rrep(i,n-1,0){
        if(ok[i]==false)continue;
        if(a[i]==-1){
            int now=i;
            while(true){
                ok[now]=false;
                if(now==0)break;
                now=(now-1)&i;
                
            }
        }
        
    }
    ll ans=-1;
    ll num=-1;
    rrep(i,n-1,0){
        if(ok[i]){
            ans=a[i]*a[i];
            num=i;
            break;
        }
    }
    if(ans==-1){
        cout<<-1<<endl;
        return 0;
    }
    ll b[n];
    fill(b,b+n,0);
    vector<int> v;
    rep(i,0,n)if(a[i]>=0)v.push_back(i);
    uniform_int_distribution<int> ra(0,v.size()-1);
    map<pii,int> mp;
    rep(i,0,100000){
        int aa=ra(mt);
        int bb=ra(mt);
        pii p={aa,bb};
        if(mp[p])continue;
        mp[p]++;
        int c=aa&bb;
        if(b[c]>=1e+19)continue;
        b[c]+=a[aa]*a[bb];
    }
    rep(i,0,n){
        if(ok[i]){
            if(b[i]>0){
                ans=gcd(ans,b[i]);
            }
        }
    }
    cout<<ans<<endl;
}