#include "bits/stdc++.h"
using namespace std;
#define int long long
#define FOR(i, a, b) for(int i=(a);i<(b);i++)
#define RFOR(i, a, b) for(int i=(b-1);i>=(a);i--)
#define REP(i, n) for(int i=0; i<(n); i++)
#define RREP(i, n) for(int i=(n-1); i>=0; i--)
#define REP1(i, n) for(int i=1; i<=(n); i++)
#define RREP1(i, n) for(int i=(n); i>=1; i--)
#define ALL(a) (a).begin(),(a).end()
#define UNIQUE_SORT(l) sort(ALL(l)); l.erase(unique(ALL(l)), l.end());
#define CONTAIN(a, b) find(ALL(a), (b)) != (a).end()
#define out(...) printf(__VA_ARGS__)
#if DEBUG
#define debug(...) printf(__VA_ARGS__)
#else
#define debug(...) /* ... */
#endif
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}

void solve();
signed main()
{
#if DEBUG
    std::ifstream in("input.txt");
    std::cin.rdbuf(in.rdbuf());
#endif
    cin.tie(0);
    ios::sync_with_stdio(false);
    solve();
    return 0;
}

/*================================*/
#if DEBUG
#define SIZE 100
#else
#define SIZE 223450
#endif

int N,Q,K;
int A[SIZE];

void solve() {
    cin>>N;
    REP(i,N)cin>>A[i];
    int ans = -1;
    REP(d,12) {
        set<int> s = {(d+0)%12, (d+2)%12, (d+4)%12, (d+5)%12, (d+7)%12, (d+9)%12, (d+11)%12};
        bool ok = true;
        REP(i,N) {
            if (s.count(A[i])==0) {
                ok = false;
            }
        }
        if (ok && ans==-1) {
            ans = d;
        }
        else if (ok && ans!=-1) {
            cout << -1 << endl;
            return;
        }
    }
    cout << ans << endl;
}