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 PyPy2 (5.6.0) (judge verdict: Accepted)

s = raw_input()
chrindex = []
for i in range(0, 26):
	chrindex.append([])
for i in range(0, len(s)):
	chrindex[ord(s[i])-97].append(i)
K = int(raw_input())
sublist = []
for i in range(0, K):
	sublist.append('')
for i in range(0, 26):
	for j in range(0, len(chrindex[i])):
		word = ''
		for k in range(chrindex[i][j], len(s)):
			word = word + s[k]
			if sublist[-1] < word and sublist[-1] != '': break
			if word not in sublist:
				for p in range(0, K):
					if word < sublist[p]:
						for q in range(0, K-p-1):
							sublist[-1-q] = sublist[-2-q]
						sublist[p] = word
						break
					if sublist[p] == '':
						sublist[p] = word
						break
	if '' not in sublist: break
print sublist[-1]