#include <algorithm>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
using namespace std;
#define rep(i, n) for(int(i) = 0; (i) < (n); (i)++)
#define FOR(i, m, n) for(int(i) = (m); (i) < (n); (i)++)
#define All(v) (v).begin(), (v).end()
#define pb push_back
#define MP(a, b) make_pair((a), (b))
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
const int INF = 1 << 30;
const ll LINF = 1LL << 60;
const int MOD = 1e9 + 7;

int main() {
    int N;
    cin >> N;
    set<int> se;
    rep(i, N) {
        int a;
        cin >> a;
        se.insert(a);
    }
    int res = -1;
    vector<int> check = {0, 2, 4, 5, 7, 9, 11};
    for(int i = 0; i < 12; i++) {
        int cnt = 0;
        for(auto c : check) {
            if(se.count((i + c) % 12))
                cnt++;
        }
        if(cnt == se.size()) {
            if(res != -1) {
                cout << -1 << endl;
                return 0;
            }
            res = i;
        }
    }
    cout << res << endl;
    return 0;
}