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

int a[200010], t[200010];

int main(){
    int n;
    cin >> n;
    for (int i = 1; i <= n; i++)
        cin >> a[i];
    sort(a+1, a+n+1);
    
    int cur = 0;
    int x = -1;
    for (int i = n; i >= 1; i--){
        if (a[i] != x){
            t[++cur] = x = a[i];
        }
    }
    if (cur < 3){
        cout << "-1\n";
        goto P;
    }
    cout << max(t[cur-2], t[cur]%t[cur-1]) << endl;
    P:
    return 0;
}