using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AtCoder
{

    static class Program
    {
        static void Main()
        {
            var S = GetStr();
            var K = GetInt();
            string[] memo = new string[K];
            for(int i = 0; i < K && i<S.Length; i++)
            {
                memo[i] = S.Substring(0, i + 1);
            }
            for(int i = 1; i < S.Length; i++)
            {
                for (int j = 1; j <= K && i + j <= S.Length; j++)
                {
                    var sub = S.Substring(i, j);
                    for (int k = 0; k < K; k++)
                    {
                        if (memo[k] == null)
                        {
                            memo[k] = sub;
                        }
                        else if (sub.CompareTo(memo[k]) == 0)
                        {
                            break;
                        }
                        else if (sub.CompareTo(memo[k]) == -1)
                        {
                            for(int l = K-1; l>k; l--)
                            {
                                memo[l] = memo[l - 1];
                            }
                            memo[k] = sub;
                            break;
                        }
                    }
                }
            }
            var ans = memo[K - 1];
            Console.WriteLine(ans);
            Console.ReadKey();
        }

        static void Solve()
        {
        }


        static public string GetStr() { return Console.ReadLine().Trim(); }
        static public int GetInt() { return int.Parse(Console.ReadLine()); }
        static public long GetLong() { return long.Parse(Console.ReadLine()); }
        static public string[] GetStrArray() { return Console.ReadLine().Split(' '); }
        static public int[] GetIntArray() { return Console.ReadLine().Split(' ').Select(int.Parse).ToArray(); }
        static public long[] GetLongArray() { return Console.ReadLine().Split(' ').Select(long.Parse).ToArray(); }
        static public char[] GetCharArray() { return Console.ReadLine().Split(' ').Select(char.Parse).ToArray(); }
        static public List<double> GetDoubleList() { return Console.ReadLine().Split(' ').Select(double.Parse).ToList(); }
        static public void WriteObjects<T>(IEnumerable<T> values) { foreach (var o in values) { Console.Write(o + " "); } }
        static string yesno(this bool b) { return b ? "yes" : "no"; }
        static string YesNo(this bool b) { return b ? "Yes" : "No"; }
        static string YESNO(this bool b) { return b ? "YES" : "NO"; }

        static bool eq<T, U>() => typeof(T).Equals(typeof(U));
        static T ct<T, U>(U a) => (T)Convert.ChangeType(a, typeof(T));
        static T cv<T>(string s) => eq<T, int>() ? ct<T, int>(int.Parse(s))
                           : eq<T, long>() ? ct<T, long>(long.Parse(s))
                           : eq<T, double>() ? ct<T, double>(double.Parse(s))
                           : eq<T, char>() ? ct<T, char>(s[0])
                                             : ct<T, string>(s);
        static void Multi<T>(out T a) => a = cv<T>(GetStr());
        static void Multi<T, U>(out T a, out U b)
        {
            var ar = GetStrArray(); a = cv<T>(ar[0]); b = cv<U>(ar[1]);
        }
        static void Multi<T, U, V>(out T a, out U b, out V c)
        {
            var ar = GetStrArray(); a = cv<T>(ar[0]); b = cv<U>(ar[1]); c = cv<V>(ar[2]);
        }
        static void Multi<T, U, V>(out T a, out U b, out V c,out V d)
        {
            var ar = GetStrArray(); a = cv<T>(ar[0]); b = cv<U>(ar[1]); c = cv<V>(ar[2]);d=cv<V>(ar[3]) ;
        }
    }
}

Measured memory_bytes: 13396