#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
  #define _GLIBCXX_DEBUG
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  rep(i, n) cin >> a[i];
  vector<int> ans(0);
  rep(x, 12) {
    set<int> s;
    s.insert(x), s.insert((x + 2) % 12), s.insert((x + 4) % 12), s.insert((x + 5) % 12), s.insert((x + 7) % 12), s.insert((x + 9) % 12), s.insert((x + 11) % 12);
    bool ok = true;
    rep(i, n) {
      if (!s.contains(a[i])) ok = false;
    }
    if (ok) ans.push_back(x);
  }
  if (ans.size() == 1) {
    cout << ans[0] << endl;
  }
  else {
    cout << -1 << endl;
  }
  return 0;
}