using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;
class Program
{
    static int NN => int.Parse(ReadLine());
    static string[] SList(long n) => Enumerable.Repeat(0, (int)n).Select(_ => ReadLine()).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var s = SList(n);
        var r = new Random();
        while (true)
        {
            var c = new char[n];
            for (var i = 0; i < n; ++i) c[i] = (char)(r.Next(2) + 'a');
            var cs = string.Concat(c);
            if (!s.Contains(cs))
            {
                WriteLine(cs);
                return;
            }
        }
    }
}