#include <bits/stdc++.h>
#define int long long
#define pai 114514
using namespace std;
int n,a[pai];
signed main(){
	cin>>n;
	for(int i = 1;i<=n;i++){
		cin>>a[i];
	}
	sort(a,a+n+1);
	n = unique(a + 1,a + 1 + n) - a - 1;
	a[0] = 0;
	if(n<=1){
		cout<<-1<<endl;
	}
	else{
		cout<<max(a[n - 2],a[n] % a[n - 1])<<endl;
	}
	return 0;
}