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

using ll=__int128;
struct T{
    ll v;
    long long cnt;
    long long bad;
};
T e=T{0,0,0};
T operator+(T lhs,T rhs){
    return T{lhs.v+rhs.v,lhs.cnt+rhs.cnt,lhs.bad+rhs.bad};
}
T operator-(T lhs,T rhs){
    return T{lhs.v-rhs.v,lhs.cnt-rhs.cnt,lhs.bad-rhs.bad};
}
T operator*(T lhs,T rhs){
    return T{lhs.v*rhs.v,lhs.cnt*rhs.cnt,lhs.bad*rhs.cnt+lhs.cnt*rhs.bad-lhs.bad*rhs.bad};
}

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;
vector<T> B(MAX_N,e);


void solve(int step,vector<T> A,int mask){
    if(A.size()==1){
        B[mask]=B[mask]+A[0]*A[0];
        return;
    }
    vector<T> odd(A.size()/2,e);
    for(int i=1;i<A.size();i+=2) odd[i/2]=A[i];
    solve(step+1,odd,mask+(1<<step));

    vector<T> even(A.size()/2,e);
    for(int i=0;i<A.size();i++) even[i/2]=even[i/2]+A[i];
    solve(step+1,even,mask);
    for(int i=0;i<A.size()/2;i++){
        int id = ((i*2)<<step)+mask;
        int id2 = ((i*2+1)<<step)+mask;
        B[id]=B[id]-B[id2];
    }
}

string to_str(ll v){
    string res;
    while(v){
        res+=('0'+v%10);
        v/=10;
    }
    reverse(res.begin(),res.end());
    return res;
}

int main(){
    int n;
    cin>>n;
    ll deb=0;
    vector<T> A(MAX_N,e);
    for(int i=0;i<n;i++){
        long long tmp;
        cin>>tmp;
        if(tmp==-1) deb++;
        if(tmp==-1) A[i]={0,1,1};
        else A[i]={tmp,1,0};
    }

    solve(0,A,0);

    vector<ll> B2(MAX_N);
    for(int i=0;i<MAX_N;i++){
        if(B[i].bad==0) B2[i]=B[i].v;
        else B2[i]=0;
    }
    long long x=0,y=0;
    for(int i=0;i<n;i++){
        x+=B[i].cnt;
        y+=B[i].bad;
    }
    
    cerr<<x<<" "<<y<<endl;
    cerr<<((long long)n)*n<<" "<<(long long)(2*deb*n-deb*deb)<<endl;
    ll ans=0;
    for(int i=0;i<n;i++){
        ans=mygcd(ans,B2[i]);
    }
    if(ans==0) cout<<-1<<endl;
    else cout<<to_str(ans)<<endl;
    
    return 0;
}