#include <bits/stdc++.h>
using namespace std;

int main()
{
	int N;
	cin >> N;
	vector<int> T( N );
	for( int i = 0; i < N; i++ ) {
		cin >> T[i];
	}

	int a[7] = { 0, 2, 4, 5, 7, 9, 11 };
	vector<vector<int>> d( N );
	for( int i = 0; i < N; i++ ) {
		for( int j = 0; j < 7; j++ ) {
			for( int dd = 0; dd < 12; dd++ ) {
				if( (dd + a[j]) % 12 == T[i] ) d[i].push_back( dd );
			}
		}
	}
	int ans = -1;
	for( int j = 0; j < d[0].size(); j++ ) {
		int dd = d[0][j];
		int flag = 0;
		for( int k = 1; k < N; k++ ) {
			if( find( d[k].begin(), d[k].end(), dd ) == d[k].end() ) {
				flag = 1;
				break;
			}
		}
		if( flag == 0 ) {
			if( ans < 0 ) ans = dd;
			else {
				ans = -1;
				break;
			}
		}
	}

	cout << ans << endl;
}