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 Ruby (2.3.3) (judge verdict: Accepted)


def add_list(list, s)
  index = 0

  while index < list.length
    if list[index].nil?
      list[index] = s
      return list
    elsif list[index] < s
      index += 1
    elsif list[index] == s
      return list
    else
      if index + 1 < list.length
        list[(index+1)..-1] = list[index..-2]
      end

      list[index] = s
      return list
    end
  end

  return list
end

s = gets.chomp
k = gets.chomp.to_i

hash = {}

s.chars.each_with_index do |c, i|
  if hash[c].nil?
    hash[c] = [i]
  else
    hash[c] << i
  end
end

char_hash = hash.sort
list = Array.new(k)

char_hash.each do |v|
  v[1].each do |i|
    k.times do |j|
      break if i+j >= s.length
      list = add_list(list, s[i..(i+j)])
    end
  end

  break if list[-1] != nil
end

puts list[-1]