/*** author: yuji9511 ***/
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using lpair = pair<ll, ll>;
const ll MOD = 1e9+7;
const ll INF = 1e18;
#define rep(i,m,n) for(ll i=(m);i<(n);i++)
#define rrep(i,m,n) for(ll i=(m);i>=(n);i--)
#define printa(x,n) for(ll i=0;i<n;i++){cout<<(x[i])<<" \n"[i==n-1];};
void print() {}
template <class H,class... T>
void print(H&& h, T&&... t){cout<<h<<" \n"[sizeof...(t)==0];print(forward<T>(t)...);}

void solve(){
    ll N;
    cin >> N;
    ll T[15];
    rep(i,0,N) cin >> T[i];
    ll d[] = {0,2,4,5,7,9,11};
    ll cnt = 0;
    ll ans = -1;

    rep(k,0,12){
        map<ll,ll> mp;
        rep(i,0,7){
            mp[(k+d[i])%12]++;
        }
        bool ok = true;
        rep(i,0,N){
            if(mp[T[i]] == 0) ok = false;
        }
        if(ok){
            cnt++;
            ans = k;
        }

    }
    if(cnt == 1){
        print(ans);
    }else{
        print(-1);
    }

}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    solve();
}