#include <iostream>
#include <set>

using namespace std;

int tyo[7] = {0,2,4,5,7,9,11};
int main(){
  int n;cin>>n;
  set<int> A;
  for(int i = 0; n > i; i++){
    int x;cin>>x;
    A.insert(x);
  }
  int ans = -1;
  for(int i = 0; 12 > i; i++){
    int z = 0;
    for(int j = 0; 7 > j; j++){
      if(A.count((i+tyo[j])%12))z++;
    }
    if(z == A.size()){
      if(ans == -1){
        ans = i;
      }else{
        cout << -1 << endl;
        return 0;
      }
    }
  }
  cout << ans << endl;
}