#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define ALL(v) v.begin(), v.end()
typedef long long ll;

#include <bits/stdc++.h>
using namespace std;

int main(){
  int n;
  cin>>n;
  
  vector<int> A(n);
  rep(i,n) cin>>A[i];
  int S[7]={0,2,4,5,7,9,11};
  int D[12];
  rep(i,12) D[i]=0;
  
  rep(d,12){
    vector<int> T=A;
    rep(j,n){
      rep(k,7){
        if(T[j]==(d+S[k])%12) T[j]=-1;
      }
    }
    bool b=true;
    rep(j,n){
      if(T[j]!=-1) b=false;
    }
    if(b) D[d]=1;
  }
  
  int cnt=0,ans;
  rep(i,12){
    if(D[i]==1){
      cnt++;
      ans=i;
    }
  }
  if(cnt==1) cout<<ans<<endl;
  else cout<<-1<<endl;
  
  return 0;
}