#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
using namespace std;
typedef long long ll;

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

    vector<int> sum = {0, 2, 4, 5, 7, 9, 11};
    int mod = 12;

    vector<int> ans;
    for (int d = 0; d < 12; d++) {
        vector<int> x(7);
        rep(i, x.size()) {
            x[i] = (d + sum[i]) % mod;
        }

        bool total_exists = true;
        rep(i, N) {
            int t = T[i];
            bool exist = false;
            rep(j, x.size()) {
                // T[i]がx[j]と同じものがあるかどうか

                if (t == x[j]) {
                    exist = true;
                    break;
                }
            }
            if (!exist) {
                total_exists = false;
                break;
            }
        }

        if (total_exists) {
            ans.push_back(d);
        }
    }

    if (ans.size() == 1) {
        cout << ans.back() << endl;
    } else {
        cout << -1 << endl;
    }
}