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

int main() {
  int64_t n;
  cin >> n;
  vector<int64_t> t(n), ans;
  for (int64_t i = 0; i < n; i++) cin >> t[i];
  int64_t d[7] = {0, 2, 4, 5, 7, 9, 11};
  for (int64_t i = 0; i < 12; i++) {
    bool flag = true;
    for (int64_t j = 0; j < n; j++) {
      bool check = false;
      for (int64_t k = 0; k < 7; k++) if (t[j] % 12 == (i + d[k]) % 12) check = true;
      if (!check) {
        flag = false;
        break;
      }
    }
    if (flag) ans.push_back(i);
  }
  if (ans.size() == 1) cout << ans[0] << '\n';
  else cout << "-1\n";
  return 0;
}