#include <bits/stdc++.h>
#define ull unsigned long long
using namespace std;
ull n,a[200005],idx=1,b[200005],now;
bool flag=false;
int main(){
    cin>>n;
    for(ull i=1;i<=n;i++) cin>>a[i];
    for(ull i=1;i<=n;i++){
        for(ull j=1;j<=n&&i!=j;j++){
            b[idx++]=a[i]%a[j];
            b[idx++]=a[j]%a[i];
        }
    }
    sort(b+1,b+idx+1,greater<int>());
    now=b[1];
    for(ull i=1;i<=idx;i++){
        if(flag){
            cout<<b[i];
            return 0;
        }
        else if(b[i]!=now) flag=true;
    }
    cout<<-1;
    return 0;
}