#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main() {
    int k;
    cin >> k;
    vector<int> arr;
    for (int i = 0; i < k; i++) {
        int t;
        cin >> t;
        arr.push_back(t);
    }
    auto ptr = unique(arr.begin(), arr.end());
    arr.erase(ptr, arr.end());
    if (arr.size() <= 2) {
        cout << -1 << endl;
        return 0;
    }
    sort(arr.begin(), arr.end());
    arr.insert(arr.begin(), 1, 0);
    auto kp = arr.size()-1;
    cout << max(arr[kp - 2], arr[kp] % arr[kp - 1]) << endl;
}