#include<cstdio>
#include<algorithm>
#include<iostream>
using namespace std;
int n, a[105], mod[10005];
int cmp(int x, int y)
{
    return x > y;
}
int main()
{
    scanf("%d", &n);
    for(int i = 1; i <= n; i++)
        scanf("%d", &a[i]);
    int cnt = 0, len;
    for(int i = 1; i <= n; i++)
        for(int j = 1; j != i && j <= n; j++)
            mod[++cnt] = a[i] % a[j];
    sort(mod + 1, mod + cnt + 1, cmp);
    len = unique(mod + 1, mod + cnt + 1) - (mod + 1);
    if(len < 2)
        puts("-1");
    else
        printf("%d", mod[2]);
    return 0;
}