using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[] NMi => ReadLine().Split().Select(c => int.Parse(c) - 1).ToArray();
    static int[][] NMap(int n) => Enumerable.Repeat(0, n).Select(_ => NMi).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var t = NList;
        var list = new HashSet<int>(new int[] { 0, 2, 4, 5, 7, 9, 11 });
        var d = -1;
        for (var i = 0; i < 12; ++i)
        {
            var flg = true;
            foreach (var ti in t)
            {
                if (!list.Contains((ti - i + 12) % 12)) flg = false;
            }
            if (flg)
            {
                if (d >= 0)
                {
                    WriteLine(-1);
                    return;
                }
                d = i;
            }
        }
        WriteLine(d);
    }
}