#include <iostream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstdlib>
#include <cassert>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <bitset>
#include <string>
#include <algorithm>
#include <utility>
#define llint long long
#define inf 1e18
#define rep(x, s, t) for(llint (x) = (s); (x) < (t); (x)++)
#define Rep(x, s, t) for(llint (x) = (s); (x) <= (t); (x)++)
#define chmin(x, y) (x) = min((x), (y))
#define chmax(x, y) (x) = max((x), (y))

using namespace std;
typedef pair<llint, llint> P;

llint n;
llint t[15];
string s = "101011010101";

int main(void)
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	cin >> n;
	for(int i = 1; i <= n; i++) cin >> t[i];
	
	llint ans = -1;
	for(int i = 0; i < 12; i++){
		bool flag = true;
		for(int j = 1; j <= n; j++){
			flag &= (s[(t[j]+12-i)%12] == '1');
		}
		if(flag){
			if(ans != -1){
				cout << -1 << endl;
				return 0;
			}
			ans = i;
		}
	}
	cout << ans << endl;
	
	return 0;
}