#include <bits/stdc++.h>
using namespace std;
vector<int> a;
vector<int> b;
int main()
{
    int n;
    cin >> n;
    a.resize(n + 1);
    for(int i = 1;i <= n;i++)   cin >> a[i];
    sort(a.begin() + 1,a.end());
    n = unique(a.begin() + 1,a.end()) - a.begin() - 1;
    for(int i = 1;i <= n;i++)
    {
        for(int j = i + 1;j <= n;j++)
        {
            b.push_back(a[i] % a[j]);
            b.push_back(a[j] % a[i]);
        }
    }
    sort(b.begin(),b.end());
    int m = unique(b.begin(),b.end()) - b.begin();
    if(m <= 2)  cout << -1 << endl;
    else
    {
        cout << b[m - 2] << endl;
    }
    return 0;
}