#include<bits/stdc++.h>
using namespace std;
int a[200005], r[1000005];
long long cnt;
int main() {
	int n, i, j;
	scanf("%d", &n);
	for (i = 1; i <= n; i++)
		scanf("%d", &a[i]);
	for (i = 1; i <= n; i++)
		for (j = 1; j <= n; j++) {
			r[cnt++] = a[i] % a[j];
		}
	sort(r, r + cnt);
	n = unique(r, r + cnt) - r;
	if (n <= 1) {
		printf("-1");
		return 0;
	} 
	printf("%d", r[n - 2]);
	return 0;
}