#include <bits/stdc++.h>
using namespace std;

struct iofast_t {
    iofast_t() {
        ios::sync_with_stdio(false);
        cin.tie(nullptr);
    }
} iofast;

struct uns_t {} uns;
template <typename Element, typename Head, typename ...Args>
auto vec(Element init, Head arg, Args ...args) {
    if constexpr (sizeof...(Args) == 0) return std::vector(arg, init);
    else return std::vector(arg, vec(init, args...));
}
template <typename Element, typename Head, typename ...Args>
auto vec(uns_t, Head arg, Args ...args) {
    return vec(Element(), arg, args...);
}

template <typename T, typename Compare = less<T>>
T &chmin(T &l, T r, Compare &&f = less<T>()) { return l = min(l, r, f); }
template <typename T, typename Compare = less<T>>
T &chmax(T &l, T r, Compare &&f = less<T>()) { return l = max(l, r, f); }

int main() {
    constexpr int mod = 12;

    constexpr int ofs[] = {
        0, 2, 4, 5, 7, 9, 11,
    };

    int n; cin >> n;

    int t[mod] = {};
    for (int i = 0; i < n; ++i) {
        int v; cin >> v; ++t[v];
    }

    auto ans = vec<int>(uns, 0);
    for (int i = 0; i < mod; ++i) {
        int sum = 0;
        for (auto v : ofs) {
            sum += t[(i + v) % mod];
        }

        if (sum == n) {
            ans.push_back(i);
        }
    }

    if (size(ans) == 1) {
        cout << ans[0] << endl;
    }
    else {
        cout << -1 << endl;
    }
}