#include <bits/stdc++.h>
#define rep(i, a, n) for (int i = a; i < n; i++)
#define repr(i, a, n) for (int i = n - 1; i >= a; i--)
using namespace std;
using ll = long long;
using P = pair<int, int>;
template <typename T> void chmin(T &a, T b) { a = min(a, b); }
template <typename T> void chmax(T &a, T b) { a = max(a, b); }

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

  int n;
  cin >> n;
  vector<int> t(n);
  rep(i, 0, n) cin >> t[i];

  vector<bool> ok(12, false);
  rep(i, 0, 12) {
    int add = 0;
    vector<int> v(7);
    rep(j, 0, 7) {
      v[j] = i + add;
      if (j == 2)
        add += 1;
      else
        add += 2;
    }

    bool outer = true;
    rep(k, 0, n) {
      bool flg = false;
      rep(j, 0, 7) {
        if (v[j] % 12 == t[k]) {
          flg = true;
          break;
        }
      }
      if (!flg) {
        outer = false;
        break;
      }
    }
    if (outer)
      ok[i] = true;
  }

  int ans = 0, count = 0;

  rep(i, 0, 12) {
    if (ok[i]) {
      count++;
      ans = i;
    }
  }

  if (count == 1)
    cout << ans << endl;
  else
    cout << -1 << endl;
}