#include <bits/stdc++.h>
using namespace std;
#define reps(i,s,n) for(int i = s; i < n; i++)
#define rep(i,n) reps(i,0,n)
#define Rreps(i,n,e) for(int i = n - 1; i >= e; --i)
#define Rrep(i,n) Rreps(i,n,0)
#define ALL(a) a.begin(), a.end()
#define fi first
#define se second
typedef long long ll;
typedef vector<ll> vec;
typedef vector<vec> mat;

ll N,M,H,W,Q,K,A,B;
string S;
//const ll MOD = 998244353;
const ll MOD = (1e+9) + 7;
typedef pair<ll, ll> P;
const ll INF = (1LL<<58);


int main() {
    cin>>N;
    vec t(N);
    vec tyo = {0, 2, 4, 5, 7, 9, 11};
    rep(i,N) cin>>t[i];
    set<ll> able;
    rep(i,12){
        rep(j,N){
            bool ok = false;
            for(ll c : tyo){
                if((i + c) % 12 == t[j] % 12) ok = true;
            }
            if(!ok) break;
            if(j == N - 1) able.insert(i);
        }
    }
    cout<<(able.size() == 1 ? *able.begin() : -1)<<endl;
}