#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
#define DUMP(x) cout << #x << " = " << (x) << endl;
#define FOR(i, m, n) for (ll i = m; i < n; i++)
#define IFOR(i, m, n) for (ll i = n - 1; i >= m; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
#define FOREACH(x, a) for (auto&(x) : (a))
#define ALL(v) (v).begin(), (v).end()
#define SZ(x) ll(x.size())

int main() {
  ll n;
  cin >> n;
  vector<ll> a(n);
  REP(i, n) { cin >> a[i]; }

  vector<ll> t = {0, 2, 4, 5, 7, 9, 11};
  vector<ll> ans;
  REP(d, 12) {
    set<ll> st;
    for (ll x : t) {
      st.insert((d + x) % 12);
    }
    bool ok = true;
    REP(i, n) {
      if (st.find(a[i]) == st.end()) {
        ok = false;
      }
    }
    if (ok) {
      ans.push_back(d);
    }
  }

  if (SZ(ans) == 1) {
    cout << ans[0] << endl;
  } else {
    cout << -1 << endl;
  }
}