#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for(ll i = 0, i##_len = (n); i < i##_len; i++)
#define reps(i, s, n) for(ll i = (s), i##_len = (n); i < i##_len; i++)
#define rrep(i, n) for(ll i = (n) - 1; i >= 0; i--)
#define rreps(i, e, n) for(ll i = (n) - 1; i >= (e); i--)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) ((ll)(x).size())
#define len(x) ((ll)(x).length())
#define endl "\n"

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    // ifstream in("input.txt");
    // cin.rdbuf(in.rdbuf());
    const ll d[] = {0, 2, 4, 5, 7, 9, 11};
    ll n;
    cin >> n;
    vector<ll> t(n);
    rep(i, n) cin >> t[i];
    set<ll> st;
    rep(i, n) st.insert(t[i]);
    ll ans = -1;
    rep(i, 12) {
        ll cnt = 0;
        rep(j, 7) {
            if (st.count((i + d[j]) % 12) > 0) {
                cnt++;
            }
        }
        if (cnt == n) {
            if (ans == -1) {
                ans = i;
            }
            else {
                cout << -1 << endl;
                return 0;
            }
        }
    }
    cout << ans << endl;
    return 0;
}