#include<iostream>
using namespace std;
int a[200005],h1=0,h2=-1,n;
int main(){
    cin>>n;
    for(int i=1;i<=n;i++)cin>>a[i];
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++){
            int md=a[i]%a[j];
            if(md==h1||md==h2)continue;
            if(md>h1){
                h2=h1;
                h1=md;
            }else if(md<h1&&md>h2){
                h2=md;
            }
        }
    }
    cout<<h2;
}