using System.Collections.Generic;
using System;

public class Hello
{
    static void Main()
    {
        var n = int.Parse(Console.ReadLine().Trim());
        string[] line = Console.ReadLine().Trim().Split(' ');
        var a = Array.ConvertAll(line, int.Parse);
        getAns(n, a);
    }
    static void getAns(int n, int[] a)
    {
        var w = new int[] { 0, 2, 4, 5, 7, 9, 11 };
        var hs = new HashSet<int>[12];
        for (int i = 0; i < 12; i++)
        {
            hs[i] = new HashSet<int>();
            foreach (var x in w) hs[i].Add((x + i) % 12);
        }
        var c = new List<int>();
        for (int i = 0; i < 12; i++)
        {
            var ok = true;
            foreach (var x in a)
            {
                if (!hs[i].Contains(x)) { ok = false; break; }
            }
            if (ok) c.Add(i);
        }
        if (c.Count == 1) Console.WriteLine(c[0]);
        else Console.WriteLine(-1);
    }
}