#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <cmath>
#include <iomanip>
#include <stack>
#include <queue>
#include <numeric>
#include <map>
#include <unordered_map>
#include <set>
#include <fstream>
#include <chrono>
#include <random>
#include <bitset>
//#include <atcoder/all>
#define rep(i,n) for(int i=0;i<(n);i++)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) ((int)(x).size())
#define pb push_back
using ll = long long;
using namespace std;
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 a, ll b) {return b?gcd(b,a%b):a;}
ll lcm(ll a, ll b) {return a/gcd(a,b)*b;}

int main(){
  int N; cin >> N;
  vector<int> A(N); rep(i,N) cin >> A[i];
  set<int> st;
  vector<int> ad = {0,2,4,5,7,9,11};
  rep(i,12){
    vector<int> v;
    rep(j,sz(ad)) v.pb((i+ad[j])%12);
    sort(all(v));
    int ok=1;
    rep(j,N){
      if(!binary_search(all(v),A[j])) ok=0;
    }
    if(ok) st.insert(i);
  }
  if(sz(st)!=1) cout << -1 << endl;
  else cout << *st.begin() << endl;
  return 0;
}