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

#define ll long long
#define rep(i, n) for (int i = 0; i < (n); i++)
#define P pair<int, int>

int main() {
  int n;
  cin >> n;
  vector<int> t(n);
  rep(i,n) cin >> t[i];

  vector<int> v = {0,2,4,5,7,9,11};

  auto check = [&](int x) {
    bool res = true;
    set<int> s;
    rep(i, v.size()) s.insert((v[i]+x)%12);

    rep(i,n) {
      if (s.count(t[i]) == 0) res = false;
    }
    return res;
  };

  vector<int> ans;
  rep(j, 12) {
    if (check(j)) ans.push_back(j);
  }
  if (ans.size() == 1) cout << ans[0] << endl;
  else cout << -1 << endl;


}