#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = false;
//---------------------------------//

int main() {
	int N;
	cin >> N;
	vector<int> T(N);
	REP(i, N) scanf("%d", &T[i]);
	vector<int> t {0, 2, 4, 5, 7, 9, 11};
	vector<int> ans;
	REP(i, 12) {
		bool ok = true;
		set<int> ss;
		REP(j, 7) ss.insert((i + t[j]) % 12);
		REP(j, N) if (!ss.count(T[j])) ok = false;
		if (ok) ans.emplace_back(i);
	}
	if (ans.size() != 1) puts("-1");
	else printf("%d\n", ans.back());
}