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

typedef long long ll;
#define REP(i,n) for(int i=0,_n=(int)(n);i<_n;++i)
#define ALL(v) (v).begin(),(v).end()
#define CLR(t,v) memset(t,(v),sizeof(t))
template<class T1,class T2>ostream& operator<<(ostream& os,const pair<T1,T2>&a){return os<<"("<<a.first<<","<<a.second<< ")";}
template<class T>void pv(T a,T b){for(T i=a;i!=b;++i)cout<<(*i)<<" ";cout<<endl;}
template<class T>void chmin(T&a,const T&b){if(a>b)a=b;}
template<class T>void chmax(T&a,const T&b){if(a<b)a=b;}

ll nextLong() { ll x; scanf("%lld", &x); return x;}

int main2() {
  int N = nextLong();
  set<int> T;
  REP(i, N) T.insert(nextLong());

  vector<int> ans;
  for (int D = 0; D < 12; D++) {
    bool ok = true;
    set<int> S;
    for (int i : {0, 2, 4, 5, 7, 9, 11}) {
      int d = (D + i) % 12;
      S.insert(d);
    }
    for (int t : T) {
      if (S.count(t) == 0) ok = false;
    }
    if (ok) ans.push_back(D);
  }

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

int main() {

#ifdef LOCAL
  for (;!cin.eof();cin>>ws)
#endif
    main2();
  return 0;
}