/**
 *   @FileName	a.cpp
 *   @Author	kanpurin
 *   @Created	2020.07.10 21:35:02
**/

#include "bits/stdc++.h" 
using namespace std; 
typedef long long ll;

int main() {
    int n;
    cin >> n;
    vector< int > t(n);
    for (int i = 0; i < n; i++) {
        cin >> t[i];
    }
    int ans = -1;
    for (int i = 0; i < 12; i++) {
        bool ok = true;
        for (int j = 0; j < n; j++) {
            if (i % 12 == t[j] || (i + 2) % 12 == t[j] || (i + 4) % 12 == t[j] || (i + 5) % 12 == t[j] || (i + 7) % 12 == t[j] || (i + 9) % 12 == t[j] || (i + 11) % 12 == t[j]) {
            }
            else {
                ok = false;
                break;
            }
        }
        if (ok && ans != -1) {
            cout << -1 << endl;
            return 0;
        }
        else if (ok) {
            ans = i;
        }
    }
    cout << ans << endl;
    return 0;
}