#include <bits/stdc++.h>
using namespace std;
set<int> c;
int a[1000010];
int main() {
	int n;
	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) {
			if (i != j) c.insert(a[i] % a[j]);
		}
	}
	int cnt = 1;
	for (auto i : c) {
		if (cnt == c.size() - 1) {
			cout << i;
			++cnt;
			break;
		}
		++cnt;
	}
	if (cnt == c.size() + 1) cout << -1;
	return 0;
}