#include <iostream>
#include <vector>

constexpr int M = 12;
const std::vector<int> ds{0, 2, 4, 5, 7, 9, 11};

template <class T>
std::vector<T> vec(int len, T elem) { return std::vector<T>(len, elem); }

void solve() {
    int n;
    std::cin >> n;

    auto judge = vec(M, vec(M, false));
    for (int i = 0; i < M; ++i) {
        for (auto d : ds) {
            judge[i][(i + d) % M] = true;
        }
    }

    std::vector<bool> result(M, true);
    while (n--) {
        int x;
        std::cin >> x;
        for (int i = 0; i < M; ++i) {
            if (!judge[i][x]) result[i] = false;
        }
    }

    std::vector<int> ans;
    for (int i = 0; i < M; ++i) {
        if (result[i]) ans.push_back(i);
    }

    std::cout << ((int)ans.size() == 1 ? ans.front() : -1) << "\n";
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}