#include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (n); i ++)
using namespace std;
using ll = long long;
using PL = pair<ll,ll>;
using P = pair<int,int>;
constexpr int INF = 1000000000;
constexpr long long HINF = 1000000000000000;
constexpr long long MOD = 1000000007;// = 998244353;
constexpr double EPS = 1e-4;
constexpr double PI = 3.14159265358979;
const int tyou[7] = {0,2,4,5,7,9,11};

int main() {
    int N; cin >> N;
    vector<int> T(N);
    rep(i,N) cin >> T[i];

    vector<int> ans;
    rep(i,12) {
        bool ok = true;
        rep(j,N) {
            bool flag = false;
            rep(k,7) {
                if ((i + tyou[k])%12 == T[j]) {
                    flag = true;
                    break;
                }
            }
            if (!flag) {
                ok = false;
                break;
            }
        }
        if (ok) ans.push_back(i);
    }
    if (ans.size() == 1) cout << ans[0] << '\n';
    else cout << -1 << '\n';
    return 0;
}