/**
*    author:  yuya1234
*    created: 10.07.2020 21:27:37
**/

#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

#define REP(i,n) for(ll i=0;i<(ll)(n);i++)
#define REPD(i,n) for(ll i=n-1;i>=0;i--)
#define FOR(i,a,b) for(ll i=a;i<=(ll)(b);i++)
#define FORD(i,a,b) for(ll i=a;i>=(ll)(b);i--)

#define ALL(x) x.begin(),x.end()
#define SORT(x) sort(x.begin(),x.end())
#define SORTD(x) sort(x.rbegin(),x.rend())
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );
#define SZ(x) ll(x.size())

#define MEMSET(v, h) memset((v), h, sizeof(v))

int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);

  int n;
  cin>>n;

  int t[n];
  REP(i,n)cin>>t[i];
  int ans=-1,cnt,aa=0;

  REP(i,12)
  {
    cnt=0;
    REP(j,n)
    {
      if((i)%12==t[j]
      || (2+i)%12==t[j]
      || (4+i)%12==t[j]
      || (5+i)%12==t[j]
      || (7+i)%12==t[j]
      || (9+i)%12==t[j]
      || (11+i)%12==t[j]) cnt++;
    }
    if(cnt==n)
    {
      ans=i;
      aa++;
    }
  }

  if(aa>1)cout<<-1<<endl;
  else cout<<ans<<endl;


  return 0;
}