#include <bits/stdc++.h>
using namespace std;
#include <math.h>
#include <iomanip>
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
const int INF=1001001001;
vector<pair<int64_t,int64_t>>prime_factorize(int64_t x){
  vector<pair<int64_t,int64_t>>p;
  for(int64_t i=2;i*i<=x;i++){
    int cnt=0;
    if(x%i==0){
    while(x%i==0){cnt++;x/=i;}
    p.push_back(make_pair(i,cnt));
    }
  }
  if(x!=1){p.push_back(make_pair(x,1));}
  return p; 
}

int main() {
  int N;
  cin>>N;
  vector<int>T(N);
  for(int i=0;i<N;i++){
    cin>>T[i];
  }
  vector<int>d={0,2,4,5,7,9,11};
  vector<int>p;
  for(int i=0;i<12;i++){
    bool flag=true;
    for(int j=0;j<N;j++){
      bool ok=false;
      for(int k=0;k<7;k++){
        if(T[j]==(i+d[k])%12){ok=true;break;;}
      }
      if(!ok){flag=false;}
    }
    if(flag){p.push_back(i);}
  }
  if(p.size()==0||p.size()>=2){cout<<-1<<endl;}
  else{cout<<p[0]<<endl;}
  return 0;
}