#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)
#define FOR(i,b,e) for (int i=(int)(b); i<(int)(e); i++)
#define ALL(x) (x).begin(), (x).end()

const double PI = acos(-1);

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  int n;
  cin >> n;

  vector<int> T(n);
  REP (i, n) cin >> T[i];

  vector<int> ret;
  vector<int> diff {0, 2, 4, 5, 7, 9, 11};
  
  REP (i, 12) {
    set<int> st;
    for (int d: diff) {
      st.insert((i + d) % 12);
    }

    bool ck = true;
    for (int t: T) {
      ck &= st.count(t);
    }

    if (ck)
      ret.push_back(i);
  }

  if (ret.size() == 1)
    cout << ret[0] << endl;
  else
    cout << -1 << endl;
  
  return 0;
}