#include <bits/stdc++.h>
#define PI 3.14159265359
using namespace std;
const int64_t mod = 1e9 + 7;

int main() {
	int N;
	cin >> N;

	vector<int> v(N);
	for (int i = 0; i < N; i++) {
		cin >> v.at(i);
		//cout << v.at(i) << endl;
	}

	int D;
	bool b;
	vector<int> v2(12);

	for (int i = 0; i < 12; i++) {
		b = true;
		int cnt = 0;

		for (int j = 0; j < N; j++) {
			int T = v.at(j);
			//cout << j << endl;

			if ((i % 12 != T) &&
				((i + 2) % 12 != T) &&
				((i + 4) % 12 != T) &&
				((i + 5) % 12 != T) &&
				((i + 7) % 12 != T) &&
				((i + 9) % 12 != T) &&
				((i + 11) % 12 != T)) {
				b = false;
				break;
			}

			if (b) cnt++;
		}

		if (b) v2.at(i) = cnt;
	}
	//cout << count(v2.begin(), v2.end(), N) << endl;

	if (count(v2.begin(), v2.end(), N) == 1) {
		//cout << 1 << endl;

		for (int i = 0; i < 12; i++) {
			if (v2.at(i) == N) {
				cout << i << endl;
				return 0;
			}
		}
	}
	else {
		cout << -1 << endl;
	}
}