#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
using P = pair<int, int>;
using PL = pair<lint, lint>;
#define FOR(i, begin, end) for(int i=(begin),i##_end_=(end);i<i##_end_;i++)
#define IFOR(i, begin, end) for(int i=(end)-1,i##_begin_=(begin);i>=i##_begin_;i--)
#define REP(i, n) FOR(i,0,n)
#define IREP(i, n) IFOR(i,0,n)
#define ALL(a)  (a).begin(),(a).end()
constexpr int MOD = 1000000007;
constexpr int INF = 2147483647;
void yes(bool expr) {
  cout << (expr ? "YES" : "NO") << "\n";
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int N;
    cin >> N;
    vector<int> T(N);
    REP(i, N) cin >> T[i];
    int ans = -1;
    REP(i, 12) {
        bool flg = true;
        REP(j, N) {
            int x = (T[j] - i + 12)%12;
            if(x == 1 || x == 3 || x == 6 || x == 8 || x == 10) {
                flg = false;
                break;
            }
        }
        if(flg) {
            if(ans != -1) {
                cout << -1 << endl;
                return 0;
            } else {
                ans = i;
            }
        }
    }
    cout << ans << endl;
}