#include <bits/stdc++.h>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;
typedef unsigned long long ull;


int main(){
  int N;
  cin >> N;
  vector<int> v;
  rep(i,N){
    int a;
    cin >> a;
    v.push_back(a);
  }

  vector<int> ret;
  rep(D,12){
    map<int,bool> ok;
    ok[D%12] = true;
    ok[(D+2)%12] = true;
    ok[(D+4)%12] = true;
    ok[(D+5)%12] = true;
    ok[(D+7)%12] = true;
    ok[(D+9)%12] = true;
    ok[(D+11)%12] = true;

    bool ck = true;
    rep(j,v.size()){
      if(ok.count(v[j]%12) == 0) ck = false;
    }
    if(ck) ret.push_back(D);
  }
  
  if(ret.size() == 1) cout << ret[0] << endl;
  else cout << -1 << endl;
  return 0;
}