#include <bits/stdc++.h>

using namespace std;

int main(void)
{
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	vector <int> add = { 0,2,4,5,7,9,11 };
	vector <int> v;
	int n, t;
	int cnt = 0;
	int res = -1;
	cin >> n;

	for (int i = 0; i < n; i++)
	{
		cin >> t;
		v.push_back(t);
	}

	for (int i = 0;i<12;i++)
	{
		bool check[12];
		memset(check, false, sizeof(check));
		for (int j = 0; j < add.size(); j++)
		{
			int val = add[j] + i;
			val %= 12;
			check[val] = true;
		}
		bool ok = true;
		for (int j = 0; j < n; j++)
		{
			if (check[v[j]] == false)
			{
				ok = false;
				break;
			}
		}
		if (ok)
		{
			cnt++;
			res = i;
		}
	}

	if (cnt > 1) res = -1;
	cout << res << '\n';
	return 0;
}