#include <bits/stdc++.h>
#define endl "\n"
using namespace std;
#define ll long long
#define ld long double
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define repo(i,n) for(int i = 1; i < (int)(n); i++)
#define pb push_back
#define mp make_pair
#define np next_permutation
#define fi first
#define se second
#define all(x) (x).begin(),(x).end()
#define uniq(v) v.erase(unique(v.begin(),v.end()),v.end())
#define lb(v,x) (lower_bound(v.begin(),v.end(),x)-v.begin())
#define ub(v,x) (upper_bound(v.begin(),v.end(),x)-v.begin())
using Pair = pair<ll,pair<int,int>>;
#define pq priority_queue<Pair, vector<Pair>, greater<Pair>> 
const ll mod=1000000007;
//const ll mod=998244353;
const ld pi=acos(-1.0);
const ll INF = 1LL<<61;
template<class T>bool chmax(T &a, const T &b) { 
  if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) {
  if (b<a) { a=b; return 1; } return 0; }
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
ll lcm(ll x, ll y) { return x / gcd(x, y) * y; }
//intの最大値2147483647 ≒ 2×10^9
//long longの最大値9223372036854775807 ≒ 9×10^18
//'大文字'+=32;    で小文字に
//  cout << fixed << setprecision (20);   小数点以下2０桁まで
//実行時間制約2秒では２×10^8回くらいまで計算できる



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

  int n;
  cin>>n;

  vector<int> p(n);
  rep(i,n){
    cin>>p[i];
  }
  
  vector<int> q={0,2,4,5,7,9,11};
  vector<vector<int>> r(12);
  rep(i,12) r[i]=q;
  rep(i,12){
    rep(j,7){
      r[i][j]+=i;
      r[i][j]%=12;
    }
  }

  vector<int> ans;

  rep(i,12){
    bool ok=true;

    rep(j,n) {
      bool aru=false;
      rep(k,7){
        if(p[j]==r[i][k]) aru=true;
      }
      if(!aru) ok=false;
    }
    if(ok) ans.pb(i);
  }

  if(ans.size()!=1) cout << -1 << endl;
  else cout << ans[0] << endl;

}