#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
using namespace std;
using ll = long long ;
using P = pair<int,int> ;
using pll = pair<long long,long long>;
constexpr int INF = 1e9;
constexpr long long LINF = 1e17;
constexpr int MOD = 1000000007;
constexpr double PI = 3.14159265358979323846;
vector<int> p = {0,2,4,5,7,9,11};

int main(){
    int n;
    cin >> n;
    vector<int> cnt(20,0);
    rep(i,n){
        int a;
        cin >> a;
        cnt[a] = 1;
    }
    int d = -1;
    rep(i,12){
        bool res = true;
        rep(j,12){
            if(cnt[j]==1){
                bool ok = false;
                rep(k,p.size()){
                    if((i+p[k])%12==j) ok = true;
                }
                if(!ok) res = false;
            }
        }
        if(res&&d!=-1){d = INF;break;}
        if(res) d = i;
    }

    if(d==INF) cout << -1 << endl;
    else cout << d << endl;

    return 0;
}