#include<bits/stdc++.h>

using namespace std;
using ll=long long;
#define rep2(i, a, n) for(int i = (a); i < (n); i++)
#define rep(i, n) rep2(i,0,n)

int main(){
  cin.tie(nullptr);ios_base::sync_with_stdio(false);
  int n;cin>>n;
  int hoge[n];rep(i,n)cin>>hoge[i];
  int shake=-1,d[7]={0,2,4,5,7,9,11},cnt;
  rep(i,12){
    cnt=0;
    rep(j,n){
      rep(k,7){
        if(hoge[j]==(i+d[k])%12)cnt++;
      }
    }
    if(cnt==n&&shake==-1){
      shake=i;
    }else if(cnt==n&&shake!=-1){
      shake=-1;
      break;
    }
  }
  cout<<(shake?shake:-1)<<endl;
}