#include "bits/stdc++.h"
using namespace std;
#define int long long
#define REP(i, n) for (int i = 0; i < (int)n; ++i)
#define RREP(i, n) for (int i = (int)n - 1; i >= 0; --i)
#define FOR(i, s, n) for (int i = s; i < (int)n; ++i)
#define RFOR(i, s, n) for (int i = (int)n - 1; i >= s; --i)
#define ALL(a) a.begin(), a.end()
#define IN(a, x, b) (a <= x && x < b)
template<class T>inline void out(T t){cout << t << "\n";}
template<class T,class... Ts>inline void out(T t,Ts... ts){cout << t << " ";out(ts...);}
template<class T>inline bool CHMIN(T&a,T b){if(a > b){a = b;return true;}return false;}
template<class T>inline bool CHMAX(T&a,T b){if(a < b){a = b;return true;}return false;}
constexpr int INF = 1e18;


signed main(){
	int N;
	cin >> N;
	vector<int>a(N);
	REP(i, N) {
		cin >> a[i];
	}
	int ans = -1;
	vector<int>b = {0, 2, 4, 5, 7, 9, 11};
	REP(i, 12) {
		int ok = 1;
		REP(j, N) {
			int flg = 0;
			for(auto e : b) {
				if(a[j] == (i + e) % 12) flg = 1;
			}
			if(flg == 0)ok = 0;
		}
		if(ok) {
			if(ans != -1) {
				out(-1);
				return 0;
			}else{
				ans = i;
			}
		}
	}
	if(ans == -1)out(-1);
	else out(ans);
}