#include<iostream>
#include<algorithm>
int a[200005];
int n;
using namespace std;
int main()
{
	cin >> n;
	int cnt = 0;
	for (int i = 1; i <= n; i++)
	{
		int x;
		cin >> x;
		if (x == a[i - 1] && cnt != 0)continue;
		a[++cnt] = x;
	}
	if (cnt < 2)
	{
		cout << -1;
		return 0;
	}
	sort(a + 1, a + cnt + 1);
	int Max2 = max(a[cnt] % a[cnt - 1], a[cnt - 2] % a[cnt]);
	cout << Max2;
	return 0;
}