#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;

int main() {
  int N;
  cin >> N;
  vector<int> T(N);
  for (int i = 0; i < N; i++) cin >> T[i];
  int cnt = 0, ans = -1;
  for (int D = 0; D < 12; D++) {
    bool flag = true;
    for (int i = 0; i < N; i++) {
      int t = T[i] % 12;
      if (t == D % 12) flag = true;
      else if (t == (D + 2) % 12) flag = true;
      else if (t == (D + 4) % 12) flag = true;
      else if (t == (D + 5) % 12) flag = true;
      else if (t == (D + 7) % 12) flag = true;
      else if (t == (D + 9) % 12) flag = true;
      else if (t == (D + 11) % 12) flag = true;
      else {
        flag = false;
        break;
      }
    }
    if (flag) ans = D, cnt++;
  }
  cout << (cnt == 1 ? ans: -1) << endl;
  return 0;
}