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: Accepted)

import java.io.IOException;
import java.io.InputStream;
import java.io.PrintStream;
import java.util.NoSuchElementException;
import java.util.PriorityQueue;
public class Main {
    private static final PrintStream ps     = System.out;
    private static final InputStream IS     = System.in;
    private static final byte[]      BUFFER = new byte[1024];
    private static int               ptr    = 0;
    private static int               buflen = 0;

    public static void main(String[] args) {
        String s = n();
        int len = s.length();
        int k = ni();
        
        PriorityQueue<String> p = new PriorityQueue<>();
        for (int i = 0; i < len; i++) {
            for (int j = 1; j <= 5; j++) {
                if (i + j > len) break;
                String sub = s.substring(i, i + j);
                
                if (!p.contains(sub))
                    p.add(sub);
            }
        }
        
        for (int i = 0; i < k-1; i++) {
            p.poll();
        }
        ps.println(p.poll());
    }
    

    private static boolean hasNextByte() {
        if (ptr < buflen)
            return true;
        else {
            ptr = 0;
            try {
                buflen = IS.read(BUFFER);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0)
                return false;
        }
        return true;
    }

    private static int readByte() {
        if (hasNextByte())
            return BUFFER[ptr++];
        else
            return -1;
    }

    private static boolean isPrintableChar(int c) {
        return 33 <= c && c <= 126;
    }

    public static boolean hasNext() {
        while (hasNextByte() && !isPrintableChar(BUFFER[ptr]))
            ptr++;
        return hasNextByte();
    }

    public static String n() {
        if (!hasNext())
            throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }

    public static long nl() {
        if (!hasNext())
            throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b)
            throw new NumberFormatException();
        while (true) {
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if (b == -1 || !isPrintableChar(b))
                return minus ? -n : n;
            else
                throw new NumberFormatException();
            b = readByte();
        }
    }

    public static int ni() {
        long nl = nl();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
            throw new NumberFormatException();
        return (int) nl;
    }

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

    private static int[] nia(int n) {
        int[] a = new int[n];
        for (int i = 0; i < n; i++)
            a[i] = ni();
        return a;
    }
}