https://www.luogu.com.cn/record/148308176

#include <iostream>
int gcd(int a, int b) {
    if(b == 0){
        return a;
    }else{
        return gcd(b, a % b);
    }
}
using namespace std;
int main() {
    int n;
    cin >> n;
    long long a[2000001];
    for(int i = 0; i < n; i++) {
        cin >> a[i];
    }
    int max_gcd = a[0];
    for (int i = 1; i < n; i++) {
        max_gcd = gcd(max_gcd, a[i]);
    }
    if(max_gcd%2!=0){
        cout << -1;
    }
    else{
        cout << max_gcd/2;
    }
    return 0;
}