typedef long long ll;
typedef long double ld;
#include <bits/stdc++.h>
using namespace std;

int main() {
    ll n;
    std::cin >> n;
    map<ll,bool> ex;
    for (int i = 0; i < n; i++) {
        ll t;
        std::cin >> t;
        ex[t] = true;
    }
    vector<ll> aa = {0,2,4,5,7,9,11};
    vector<ll> ans;
    for (int i = 0; i < 12; i++) {
        bool ok = true;
        for (int j = 0; j < 12; j++) {
            if(ex[j]){
                bool okk = false;
                for (auto e : aa) {
                    if(j==(e+i)%12){
                        okk = true;
                    }
                }
                if(!okk){
                    ok = false;
                    break;
                }
            }
        }
        if(ok) ans.push_back(i);
    }
    
    if(ans.size()==1){
        std::cout << ans[0] << std::endl;
    }else{
        std::cout << -1 << std::endl;
    }
}