#include <cstdio>
#include <iostream>
#include <string>
#include <sstream>
#include <stack>
#include <algorithm>
#include <cmath>
#include <queue>
#include <map>
#include <set>
#include <cstdlib>
#include <bitset>
#include <tuple>
#include <assert.h>
#include <deque>
#include <bitset>
#include <iomanip>
#include <limits>
#include <chrono>
#include <random>
#include <array>
#include <unordered_map>
#include <functional>
#include <complex>
#include <numeric>
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; }
constexpr long long MAX = 5100000;
constexpr long long INF = 1LL << 60;
constexpr int inf = 1000000007;
constexpr long long mod = 1000000007LL;
//constexpr long long mod = 998244353LL;
const long double PI = acos((long double)(-1));

using namespace std;
typedef unsigned long long ull;
typedef long long ll;
typedef long double ld;

int main()
{
	/*
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	*/

	int n; scanf("%d", &n);
	vector<int> t(n); for (int i = 0; i < n; i++) scanf("%d", &t[i]);
	vector<int> res;
	vector<int> d = { 0,2,4,5,7,9,11 };
	for (int D = 0; D < 12; D++) {
		bool ok = true;
		for (int i = 0; i < n; i++) {
			bool f = false;
			for (auto e : d) {
				if (t[i] == (e + D) % 12) f = true;
			}
			if (!f) ok = false;
		}
		if (ok) res.emplace_back(D);
	}
	if (res.size() == 1) cout << res[0] << endl;
	else cout << -1 << endl;
	return 0;
}