#include<bits/stdc++.h>
using namespace std;
#define int long long
#define ii pair <int, int>
#define app push_back
#define all(a) a.begin(), a.end()
#define bp __builtin_popcountll
#define ll long long
#define mp make_pair
#define f first
#define s second
#define Time (double)clock()/CLOCKS_PER_SEC
#define debug(x) std::cout << #x << ": " << x << '\n';
signed main() {
    #ifdef HOME
    freopen("input.txt", "r", stdin);
    #else
    #define endl '\n'
    ios_base::sync_with_stdio(0); cin.tie(0);
    #endif

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

    set <int> s[12];
    for (int i = 0; i < 12; ++i) {
        for (int j : sh)
            s[i].insert((i+j)%12);
    }   

    int n;
    cin >> n;
    vector <bool> can(12, 1);
    while (n--) {
        int x;
        cin >> x;

        for (int i = 0; i < 12; ++i)    
            if (s[i].find(x) == s[i].end())
                can[i] = 0;

    }
    vector <int> ans;
    for (int i = 0; i < 12; ++i)
        if (can[i])
            ans.app(i);
    if (ans.size() != 1)
        cout << -1 << endl;
    else
        cout << ans[0] << endl;
}