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

using ll = long long;
using ull = unsigned long long;
using ld = long double;
template<class t> using vc = vector<t>;
template<class t> using vvc = vc<vc<t>>;
using pi = pair<int,int>;
using vi = vc<int>;
using vvi = vvc<int>;

#define rep(i,a,b) for (int i = a; i < b; i++)
#define irep(i,a,b) for (int i = a; i > b; i--)
#define print(n) cout << n << endl
#define rup(a,b) (a+b-1)/b


int main(){
    cout << fixed << setprecision(15);
    
    int N;
    cin>>N;
    int T[N];
    rep(i,0,N) cin>>T[i];
    int now = -1;
    bool p = false;
    int d[]={0,2,4,5,7,9,11};
    int k[7];
    rep(i,0,12){
        rep(j,0,7){
            k[j] = (i+d[j]) % 12;
        }
        bool qq = true;
        rep(j,0,N){
            bool q = false;
            rep(a,0,7){
                if(T[j]==k[a]){
                    q = true;
                    break;
                }
            }
            if (q) continue;
            else {
                qq = false;
                break;}
        }
        if(qq){
            if(!p){
                now = i;
                p = true;
                continue;
            }else{
                print(-1);
                return 0;
            }
        }
    }
    print(now);

    
    //system("pause");
    return 0;
}