#pragma GCC optimize ("O2")
#pragma GCC target ("avx2")
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for(int i = 0; i < (n); i++)
#define rep1(i, n) for(int i = 1; i <= (n); i++)
#define co(x) cout << (x) << "\n"
#define cosp(x) cout << (x) << " "
#define ce(x) cerr << (x) << "\n"
#define cesp(x) cerr << (x) << " "
#define pb push_back
#define mp make_pair
#define chmin(x, y) x = min(x, y)
#define chmax(x, y) x = max(x, y)
#define Would
#define you
#define please


int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);


	int chou[12] = { 1,0,1,0,1,1,0,1,0,1,0,1 };

	int N;
	cin >> N;
	int T[12];
	rep(i, N) cin >> T[i];

	vector<int> kotae;
	rep(i, 12) {
		int OK = 1;
		rep(j, N) {
			if (!chou[(T[j] - i + 12) % 12]) OK = 0;
		}
		if (OK) kotae.pb(i);
	}

	if (kotae.size() == 0 || kotae.size() > 1) {
		co(-1);
	}
	else co(kotae[0]);



	Would you please return 0;
}