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

public static partial class Program
{
    public static void Main()
    {
        var str = CRL();
        var k = CRL().ToInt();
        var list = new List<string>();
        for (int len = 1; len <= Math.Min(k, str.Length); len++)
        {
            for (int start = 0; start <= str.Length - len; start++)
            {
                var sb = new StringBuilder();
                for (int point = 0; point < len; point++)
                {
                    sb.Append(str[start + point]);
                }
                list.Add(sb.ToString());
            }
        }

        list = list.Distinct().OrderBy(x => x).ToList();
        CWL(list[k - 1]);
    }
}

public static partial class Program
{
    //テンプレここから
    public static long GCD(long a, long b) => b == 0 ? a : GCD(b, a % b);
    public static int GCD(int a, int b) => b == 0 ? a : GCD(b, a % b);
    public static long LCM(long a, long b) => a * b / GCD(a, b);
    public static int LCM(int a, int b) => a * b / GCD(a, b);
    public static int ToInt(this double d) => (int) d;
    public static long ToLong(this double d) => (long) d;
    public static int ToInt(this string str) => int.Parse(str);
    public static long ToLong(this string str) => long.Parse(str);
    public static long[] ToLong(this IEnumerable<string> strs) => strs.Select(x => x.ToLong()).ToArray();
    public static int[] ToInt(this IEnumerable<string> strs) => strs.Select(x => x.ToInt()).ToArray();
    public static long[] ArrayLongs(this string str, char opr = ' ') => str.Split(opr).ToLong();
    public static int[] ArrayInts(this string str, char opr = ' ') => str.Split(opr).ToInt();
    public static string CRL() => Console.ReadLine();
    public static string RL() => ReadLine();
    public static void CWL(object obj) => Console.WriteLine(obj);
    public static void WL(object obj) => WriteLine(obj);
    public static long Diff(long a, long b) => Math.Abs(Math.Max(a, b) - Math.Min(a, b));
    public static int Diff(int a, int b) => Math.Abs(Math.Max(a, b) - Math.Min(a, b));
    public static long Sign(this long i) => Math.Sign(i);
    public static bool IsEven(this long i) => i % 2 == 0;
    public static bool IsEven(this int i) => i % 2 == 0;
    public static long[] dx = new long[] {0, -1, 0, 1};
    public static long[] dy = new long[] {-1, 0, 1, 0};
    public static long mod = 1000000007;
    public static string alphabet = "abcdefghijklmnopqrstuvwxyz";
    public static long ToLong(this char c) => c.ToString().ToInt();
    public static long[] Sort(this long[] array) => array.OrderBy(x => x).ToArray();
    public static int[] Sort(this int[] array) => array.OrderBy(x => x).ToArray();
    public static long[] DSort(this long[] array) => array.OrderByDescending(x => x).ToArray();
    public static int[] DSort(this int[] array) => array.OrderByDescending(x => x).ToArray();
    public static string Join(this IEnumerable<string> array, string sep = "") => string.Join(sep, array);

    public static string Join(this IEnumerable<char> array, string sep = "") => string.Join(sep, array);

    //テンプレここまで
}


Measured memory_bytes: 17888