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 Scala (2.11.7) (judge verdict: Time Limit Exceeded)

import scala.io.StdIn

object Main extends App {

  val s = StdIn.readLine()
  val K = StdIn.readLine().toInt
  
  //val m = scala.collection.mutable.Set.empty[String]
  val m = scala.collection.mutable.SortedSet.empty[String]
  
  // i <- 0 until s.size
  //   j <- 1 until s.size - i
  for(i <- 0 until s.size;
      j <- 1 to s.size - i){
    //println(s"$i $j")
    val temp = s.substring(i, i + j)
    if(temp.size <= K && !m.contains(temp)){
      m add temp
    }
  }
  
  //println(m.mkString(" "))
  def getK(t: scala.collection.mutable.SortedSet[String], index: Int): String = {
    if(K == index) t.head
    else getK(t.tail, index + 1)
  }
  println(getK(m, 1))
}