#include <bits/stdc++.h>

using ll = long long;

using namespace std;

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

    int n;
    scanf("%d", &n);

    vector<int> s0{0,2,4,5,7,9,11};
    vector<int> t(n);
    for (int i = 0; i < n; ++i)
        scanf("%d", &t[i]);

    sort(t.begin(), t.end());
    int res{ -1 };
    vector<int> s(s0);
    for (int i = 0; i < 12; ++i)
    {
        if (includes(s.begin(), s.end(), t.begin(), t.end()))
        {
            if (res != -1)
            {
                res = -1;
                break;
            }
            res = i;
        }

        s.clear();
        for (size_t j = 0; j < s0.size(); ++j)
            s.push_back((s0[j] + i + 1) % 12);

        sort(s.begin(), s.end());
    }

    printf("%d\n",res);

    return 0;
}