#include<bits/stdc++.h>
using namespace std;
#define int long long
#define REP(i,m,n) for(int i=(m);i<(n);i++)
#define rep(i,n) REP(i,0,n)
#define pb push_back
#define all(a) a.begin(),a.end()
#define rall(c) (c).rbegin(),(c).rend()
#define mp make_pair
#define endl '\n'
typedef long long ll;
typedef pair<ll,ll> pll;
typedef long double ld;
const ll inf=1e9+7;
const ll mod=1e9+7;
signed main(){
    ll n;cin>>n;
    vector<ll>t(n);
    rep(i,n)cin>>t[i];
    ll ans=-1;
    ll cnt=0;
    vector<ll>a(100);
    a[0]=1;
    a[2]=1;
    a[4]=1;
    a[5]=1;
    a[7]=1;
    a[9]=1;
    a[11]=1;
    rep(i,12){
        bool f=1;
        rep(j,n){
            bool tt=0;
            rep(k,20){
                if(a[k]){
                    if((k+i)%12==t[j])tt=1;
                }
            }
            f&=tt;
        }
        if(f){
            ans=i;
            cnt++;
        }
    }
    if(cnt>1){
        ans=-1;
    }
    cout<<ans<<endl;
}