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 Go (1.6) (judge verdict: Time Limit Exceeded)

package main

import (
        "fmt"
        "sort"
)

func main() {
        var s string
        var k int
        fmt.Scanln(&s)
        fmt.Scan(&k)

        ss := make([]string, 0, 5)
        for l := 1; l <= len(s); l++ {
                for i := 0; i < len(s)-l+1; i++ {
                        ss = append(ss, s[i:i+l])
                }
        }
        sort.Strings(ss)

        cnt := 0
        for i := range ss {
                if i >= 1 && ss[i] == ss[i-1] {
                        continue
                }
                cnt++
                if cnt == k {
                        fmt.Println(ss[i])
                        return
                }
        }
}