#include <bits/stdc++.h>
using namespace std;
typedef signed long long ll;

#undef _P
#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define ITR(x,c) for(__typeof(c.begin()) x=c.begin();x!=c.end();x++)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
//-------------------------------------------------------

int N;
ll A[1<<18];
__int128 B[1<<18];


void solve() {
	int i,j,k,l,r,x,y; string s;
	
	cin>>N;
	FOR(i,N) {
		cin>>A[i];
		if(A[i]==-1) A[i]=0;
	}
	
	FOR(i,18) FOR(j,1<<18) if((j&(1<<i))==0) A[j]+=A[j^(1<<i)];
	
	FOR(i,1<<18) {
		B[i]=A[i];
		B[i]*=B[i];
	}
	
	FOR(i,18) FOR(j,1<<18) if((j&(1<<i))==0) B[j]-=B[j^(1<<i)];
	__int128 T=0;
	FOR(i,1<<18) T=__gcd(T,B[i]);
	if(T==0) T=-1;
	cout<<(ll)T<<endl;
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false), cin.tie(0);
	FOR(i,argc-1) s+=argv[i+1],s+='\n'; FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	cout.tie(0); solve(); return 0;
}