Kth Substring
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 = x1x2...xn and Y = y1y2...ym be two distinct strings. X is lexicographically larger than Y if and only if Y is a prefix of X or xj > yj where j is the smallest integer such that xj ≠ yj.
Constraints
- 1 ≤ |s| ≤ 5000
- s consists of lowercase English letters.
- 1 ≤ K ≤ 5
- s has at least K different substrings.
Partial Score
- 67 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