#include<bits/stdc++.h>
using namespace std;
#include<boost/multiprecision/cpp_int.hpp>
namespace mp=boost::multiprecision;
using Int=mp::cpp_int;

using ll=Int;
ll mygcd(ll a,ll b){
    return b==0 ? a : mygcd(b,a%b);
}

const int LOG_N=18;
const int MAX_N=1<<LOG_N;
ll B[MAX_N];

ll cross(ll a,ll b){
   return min(a,b)==-1LL ? ll(-1) : a*b;
}
void add(ll& lhs,ll v){
    if(min(lhs,v)==-1) lhs=-1;
    else lhs+=v;
}
void solve(int step,vector<ll> A,int mask){
    if(A.size()==1){
        add(B[mask],cross(A[0],A[0]));
        return;
    }
    vector<ll> odd(A.size()/2);
    for(int i=1;i<A.size();i+=2) odd[i/2]=A[i];
    solve(step+1,odd,mask+(1<<step));

    vector<ll> even(A.size()/2);
    for(int i=0;i<A.size();i++) add(even[i/2],A[i]);
    solve(step+1,even,mask);
}

int main(){
    int n;
    cin>>n;
    vector<ll> A(MAX_N);
    for(int i=0;i<n;i++){
        int tmp;
        cin>>tmp;
        A[i]=tmp;
    }

    solve(0,A,0);

    for(int i=0;i<MAX_N;i++){
        if(B[i]==-1) B[i]=0;
    }

    ll ans=0;
    for(int i=0;i<MAX_N;i++){
        ans=mygcd(ans,B[i]);
    }
    if(ans==0) cout<<-1<<endl;
    else cout<<ans<<endl;
    
    return 0;
}