#include <bits/stdc++.h>
using namespace std;
template <class T> inline bool chmax(T &a, T b) {
    if(a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <class T> inline bool chmin(T &a, T b) {
    if(a > b) {
        a = b;
        return 1;
    }
    return 0;
}
void debug() { cerr << "\n"; }
template <class T> void debug(const T &x) { cerr << x << "\n"; }
template <class T, class... Args> void debug(const T &x, const Args &... args) {
    cerr << x << " ";
    debug(args...);
}
template <class T> void debugVector(const vector<T> &v) {
    for(const T &x : v) {
        cerr << x << " ";
    }
    cerr << "\n";
}
using ll = long long;

#define ALL(v) (v).begin(), (v).end()
#define RALL(v) (v).rbegin(), (v).rend()
const double EPS = 1e-7;
const int INF = 1 << 30;
const ll LLINF = 1LL << 60;
const double PI = acos(-1);
constexpr int MOD = 1000000007;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

//-------------------------------------

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

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n;
    cin >> n;
    vector<int> t(n);
    for(int i = 0; i < n; i++) {
        cin >> t[i];
    }
    vector<int> v;
    for(int d = 0; d < 12; d++) {
        bool ok = 1;
        for(int i = 0; i < n; i++) {
            bool ok2 = 0;
            for(int j = 0; j < 7; j++) {
                if((d + f[j]) % 12 == t[i]) {
                    ok2 = 1;
                }
            }
            if(!ok2) {
                ok = 0;
            }
        }
        if(ok) {
            v.push_back(d);
        }
    }
    if(v.size() == 1) {
        cout << v[0] << endl;
    } else {
        cout << -1 << endl;
    }
}