#include <iostream>
#include <string>
#include <cstring>
#include <algorithm>
#include <stack>
#include <queue>
#include <vector>
#include <deque>
#include <set>
#include <map>
#include <iomanip>
#include <cmath>

using namespace std;

const int inf = 1061109567;

set <int> val;
set <int, greater<int>> s;
int n;

int main()
{
    cin >> n;
    for (int i = 1; i <= n; ++ i)
    {
        int x;
        cin >> x;
        val.insert(x);
    }
    for (auto i = val.begin(); i != val.end(); ++ i)
    {
        auto j = ++ i;
        i --;
        for (; j != val.end(); ++ j) s.insert(*i % *j);
    }
    if (s.size() < 2)
    {
        cout << -1 << endl;
        exit(0);
    }
    auto i = s.begin();
    cout << *(++ i) << endl;

    return 0;
}