#include <bits/stdc++.h>

typedef long long ll;
const int maxn = 1e6 + 3;

int n, m;
ll a[maxn];

void fwt() {
    for(int i = 1; i < m; i *= 2)
    for(int j = 0; j < m; j += i * 2)
    for(int k = 0; k < i; ++k)
        a[j + k] += a[j + k + i];
}

void ifwt() {
    for(int i = 1; i < m; i *= 2)
    for(int j = 0; j < m; j += i * 2)
    for(int k = 0; k < i; ++k)
        a[j + k] -= a[j + k + i];
}

int main() {
    scanf("%d", &n);
    m = 1;
    while(m < n)
        m *= 2;
    for(int i = 0; i < n; ++i) {
        scanf("%lld", a + i);
        if(a[i] == -1)
            a[i] = 0;
    }
    fwt();
    for(int i = 0; i < m; ++i)
        a[i] *= a[i];
    ifwt();
    ll ans = 0;
    for(int i = 0; i < n; ++i)
        ans = std::__gcd(ans, a[i]);
    printf("%lld\n", ans ? ans : -1);
    return 0;
}