#include<iostream>
#include<algorithm>
#define maxn 200010
using namespace std;
int main(){
    int n,a[maxn];
    int b[1000000010];
    for(int i=1;i<=n;i++)cin>>a[i];
    sort(a+1,a+n+1);
    int d=1;
    for(int i=1;i<=n;i++){
        if(b[a[i]]==0){
            a[d]=a[i];
            b[a[i]]++;
            d++;
        }
    }
    if(d<3)cout<<"-1";
    else cout<<max(a[d-2],a[d-1]%a[d]);
    return 0;
}