#pragma GCC optimize("O3")
#include <bits/stdc++.h>
#define ll long long
#define rep(i,n) for(ll i=0;i<(n);i++)
#define pll pair<ll,ll>
#define pq priority_queue
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define ios ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
#define lb(c,x) distance(c.begin(),lower_bound(all(c),x))
#define ub(c,x) distance(c.begin(),upper_bound(all(c),x))

using namespace std;

template<class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
template<class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;}return 0;}

const ll mod=1e9+7;

int main() {
    ll n;
    cin >> n;
    vector<ll> t(n),ans;
    rep(i,n){
        cin >> t[i];
    }
    ll d[7]={0,2,4,5,7,9,11};
    rep(i,12){
        bool ok=true;
        rep(j,n){
            bool ex=false;
            rep(k,7){
                if(t[j]%12==(i+d[k])%12) ex=true;
            }
            if(!ex){
                ok=false;
                break;
            }
        }
        if(ok){
            ans.push_back(i);
        }
    }
    if(ans.size()==1) cout << ans[0] << endl;
    else cout << -1 << endl;
    return 0;
}