#define _USE_MATH_DEFINES
#include  <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <string>
#include<math.h>
#include<iomanip>
#include<stdio.h>
#include <stdlib.h>
#include<stdio.h>
#include <queue>
#include<map>
#include <fstream>
#include <sstream>
#include<set>
#include<stack>
#include<time.h>
#include <sys/types.h>
#include <tuple>
#include <numeric>
#include <iterator>

typedef long long int ll;
//#include<bits/stdc++.h>


using namespace std;

ll gcd(ll a, ll b) {
    if (a < b)swap(a, b);
    if (b == 0)return a;
    return gcd(b, a % b);
}
int lcm(int a, int b) {
    return a * b / gcd(a, b);
}


int main()
{

    int Q;
    Q = 1;
    //cin >> Q;
    
    while (Q--) {

        int n;
        cin >> n;

        vector<int>p(n);
        for (int i = 0; i < n; i++)cin >> p[i];
        vector<int>a;
        for (int i = 0; i < 12; i++) {
            bool f = 1;
            for (int j = 0; j < n; j++) {
                if (p[j] == i)continue;
                if ((i + 2) % 12 == p[j])continue;
                if ((i + 4) % 12 == p[j])continue;
                if ((i + 5) % 12 == p[j])continue;
                if ((i + 7) % 12 == p[j])continue;
                if ((i + 9) % 12 == p[j])continue;
                if ((i + 11) % 12 == p[j])continue;
                f = 0;
            }
            if (f)a.push_back(i);
        }
        if (a.size() != 1)cout << -1;
        else cout << a[0];

        }
    return 0;
}