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


int main() {
    int n;
    cin >> n;
    
    vector<int> t(n);
    for (int i = 0; i < n; i++) {
        cin >> t.at(i);
    }
    
    if (n <= 2 || n >= 8) {
        cout << -1 << endl;
    }
    
    else {
        int answer = -1;
        bool first = false;
        bool second = false;
        for (int i = 0; i < 12; i++) {
            for (int j = 0; j < n; j++) {
                int x = (t.at(j) - i + 12) % 12;
                if (x != 0 && x != 2  && x != 4 && x != 5 && x != 7 && x != 9 && x != 11) {
                    break;
                }
                else if (j == n - 1) {
                    if (first) {
                        second = true;
                    }
                    answer = i;
                    first = true;
                }
            }
            if (second) {
                cout << -1 << endl;
                break;
            }
            else if (i == 11) {
                cout << answer << endl;
            }
        }
    }
}