#include<bits/stdc++.h>
using namespace std;
int a[200010],n,k,l;
bool cmp(int x,int y){
	return x>y;
}
int main(){
	cin>>n;
	for(int i=1;i<=n;i++) cin>>a[i];
	sort(a+1,a+1+n,cmp);
	k=2;
	while(k<=n&&a[k]==a[k-1]) k++;
	l=k+1;
	while(l<=n&&a[l]==a[l-1]) l++;
	if(k>n) cout<<-1;
	else if(l>n) cout<<a[l]%a[k];
	else cout<<max(a[l],a[l]%a[k]);
	return 0;
}