Problem:
Score : 300 points

Problem Statement

You are given a string s.
Among the different substrings of s, print the K-th lexicographically smallest one.

A substring of s is a string obtained by taking out a non-empty contiguous part in s.
For example, if s = ababc, a, bab and ababc are substrings of s, while ac, z and an empty string are not.
Also, we say that substrings are different when they are different as strings.

Let X = x_{1}x_{2}...x_{n} and Y = y_{1}y_{2}...y_{m} be two distinct strings. X is lexicographically larger than Y if and only if Y is a prefix of X or x_{j} > y_{j} where j is the smallest integer such that x_{j} \neq y_{j}.

Constraints

1 ≤ |s| ≤ 5000

s consists of lowercase English letters.

1 ≤ K ≤ 5

s has at least K different substrings.

Partial Score

200 points will be awarded as a partial score for passing the test set satisfying |s| ≤ 50.

Input

Input is given from Standard Input in the following format:

s
K

Output

Print the K-th lexicographically smallest substring of K.

Sample Input 1

aba
4

Sample Output 1

b

s has five substrings: a, b, ab, ba and aba.
Among them, we should print the fourth smallest one, b.
Note that we do not count a twice.

Sample Input 2

atcoderandatcodeer
5

Sample Output 2

andat

Sample Input 3

z
1

Sample Output 3

z

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Time Limit Exceeded)

import java.io.*;
import java.util.*;

/**
 *
 * @author baito
 */
public class Main
{
    static StringBuilder sb = new StringBuilder();
    static FastScanner sc = new FastScanner(System.in);
    static long MOD =1000000007;
    static long[] f;//factorial

    static int N,K;
    static String s;
    static Set<String> str;

    public static void main(String[] args)
    {
        s = sc.next();
        K = sc.nextInt();
        int slen = s.length();
        str = new TreeSet<>();
        //longを忘れるなオーバーフローするぞ
        for (int len = 1; len <= slen ; len++)
        {
            for (int i = 0; i + len  <= slen ; i++)
            {
                str.add(s.substring(i,i + len));
            }
        }
        int cou = 0;
        for (String s1 : str)
        {
            cou ++;
            if(cou == K){
                System.out.println(s1);
                return;
            }
        }



    }
    //↓nCrをmod計算するために必要。　***factorial(N)を呼ぶ必要がある***
    static long comb(int n, int r)
    {
        long result = f[n];
        result = result * modInv(f[n - r]) % MOD;
        result = result * modInv(f[r]) % MOD;
        return result;
    }

    static long modInv(long n)
    {
        return modPow(n, MOD - 2);
    }

    static void factorial(int n)
    {
        f = new long[n + 1];
        f[0] = f[1] = 1;
        for (int i = 2; i <= n; i++)
        {
            f[i] = (f[i - 1] * i) % MOD;
        }
    }

    static long modPow(long x, long n)
    {
        long res = 1L;
        while (n > 0)
        {
            if ((n & 1) == 1)
            {
                res = res * x % MOD;
            }
            x = x * x % MOD;
            n >>= 1;
        }
        return res;
    }

    //↑nCrをmod計算するために必要

    static int gcd(int n, int r) { return r == 0 ? n : gcd(r, n%r); }
    static long gcd(long n, long r) { return r == 0 ? n : gcd(r, n%r); }

    static <T> void swap(T[] x, int i, int j) { T t = x[i]; x[i] = x[j]; x[j] = t; }
    static void swap(int[] x, int i, int j) { int t = x[i]; x[i] = x[j]; x[j] = t; }
    public static void reverse(int[] x)
    {
        int l = 0;
        int r = x.length - 1;
        while (l < r)
        {
            int temp = x[l];
            x[l] = x[r];
            x[r] = temp;
            l++;
            r--;
        }
    }
    public static void reverse(int[] x,int s, int e)
    {
        int l = s;
        int r = e;
        while (l < r)
        {
            int temp = x[l];
            x[l] = x[r];
            x[r] = temp;
            l++;
            r--;
        }
    }
    static int length(int a) {int cou = 0; while(a != 0){ a /= 10; cou++; } return cou;}
    static int length(long a) {int cou = 0; while(a != 0){ a /= 10; cou++; } return cou;}
    static int countC2(char[][] a, char c){
        int co = 0;
        for (int i = 0; i < a.length; i++)
            for (int j = 0; j < a[0].length; j++)
                if(a[i][j] == c)
                    co++;
        return co;
    }
    static void fill(int[][] a, int v){
        for (int i = 0; i < a.length; i++)
            for (int j = 0; j < a[0].length; j++)
                a[i][j] = v;
    }
    static class FastScanner {

        private BufferedReader reader = null;
        private StringTokenizer tokenizer = null;

        public FastScanner(InputStream in) {
            reader = new BufferedReader(new InputStreamReader(in));
            tokenizer = null;
        }

        public String next() {
            if (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }
        /*public String nextChar(){
            return (char)next()[0];
        }*/
        public String nextLine() {
            if (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    return reader.readLine();
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }

            return tokenizer.nextToken("\n");
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

        public int[] nextIntArray(int n) {
            int[] a = new int[n];
            for (int i = 0; i < n; i++) {
                a[i] = nextInt();
            }
            return a;
        }
        public int [][] nextIntArray2(int h, int w){
            int[][] a = new int[h][w];
            for(int hi = 0 ; hi < h ; hi++){
                for(int wi = 0 ; wi < w ; wi++){
                    a[hi][wi] = nextInt();
                }
            }
            return a;
        }
        public Integer[] nextIntegerArray(int n) {
            Integer[] a = new Integer[n];
            for (int i = 0; i < n; i++) {
                a[i] = nextInt();
            }
            return a;
        }
        public char[] nextCharArray(int n){
            char[] a = next().toCharArray();

            return a;
        }
        public char[][] nextCharArray2(int h , int w){
            char[][] a = new char[h][w];
            for (int i = 0; i < h; i++) {
                a[i] = next().toCharArray();
            }
            return a;
        }
        //スペースが入っている場合
        public char[][] nextCharArray2s(int h , int w){
            char[][] a = new char[h][w];
            for (int i = 0; i < h; i++) {
                a[i] = nextLine().replace(" ","").toCharArray();
            }
            return a;
        }
        public char[][] nextWrapCharArray2(int h , int w, char c){
            char[][] a = new char[h + 2][w + 2];
            //char c = '*';
            int i;
            for (i = 0; i < w + 2; i++)
                a[0][i] = c;
            for (i = 1; i < h + 1; i++) {
                a[i] = (c + next() + c).toCharArray();
            }
            for (i = 0; i < w + 2; i++)
                a[h + 1][i] = c;
            return a;
        }
        //スペースが入ってる時用
        public char[][] nextWrapCharArray2s(int h , int w ,char c){
            char[][] a = new char[h + 2][w + 2];
            //char c = '*';
            int i;
            for (i = 0; i < w + 2; i++)
                a[0][i] = c;
            for (i = 1; i < h + 1; i++) {
                a[i] = (c + nextLine().replace(" ","") + c).toCharArray();
            }
            for (i = 0; i < w + 2; i++)
                a[h + 1][i] = c;
            return a;
        }
        public long[] nextLongArray(int n) {
            long[] a = new long[n];
            for (int i = 0; i < n; i++) {
                a[i] = nextLong();
            }
            return a;
        }
        public long [][] nextLongArray2(int h, int w){
            long[][] a = new long[h][w];
            for(int hi = 0 ; hi < h ; hi++){
                for(int wi = 0 ; wi < h ; wi++){
                    a[h][w] = nextLong();
                }
            }
            return a;
        }
    }
}