#include <bits/stdc++.h>
using namespace std;

bool iscenter(const int i, const vector<int> &t) {
    for (auto x : t) {
        switch ((x - i + 12) % 12) {
            case 1:
            case 3:
            case 6:
            case 8:
            case 10:
                return false;
        }
    }
    return true;
}

int main() {
    int n;
    cin >> n;
    assert(1 <= n && n <= 12);
    vector<int> t(n);
    for (int i = 0; i < n; i++) {
        cin >> t.at(i);
        assert(0 <= t.at(i) && t.at(i) < 12);
        if (i > 0) assert(t.at(i) > t.at(i - 1));
    }
    int ok = -1;
    for (int i = 0; i < 12; i++) {
        if (iscenter(i, t)) {
            if (ok == -1) {
                ok = i;
            } else {
                cout << -1 << endl;
                return 0;
            }
        }
    }
    cout << ok << endl;
}