// // Created by yamunaku on 2020/02/11. // #include 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"<; using mti = vector>; using vl = vector; using mtl = vector>; using pi = pair; using pl = pair; template using heap = priority_queue, function>; 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; assert(b[i] == 0); 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; }