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 Python (3.4.3) (judge verdict: Accepted)

s = list(input())
K = int(input())
n = len(s)

s1 = list(set(s.copy()))
s1.sort()
for i in range(5):
  s.append('')

if(len(s1) >=3):  n1 = 3
else:  n1 = len(s1)
first = []
for i in range(n1):
  first.append(s1[i])

s2 = []
for i in range(n):
  for j in first:
    if(s[i] == j):
      s2.append(s[i]),s2.append(s[i]+s[i+1]),s2.append(s[i]+s[i+1]+s[i+2])
      s2.append(s[i]+s[i+1]+s[i+2]+s[i+3]),s2.append(s[i]+s[i+1]+s[i+2]+s[i+3]+s[i+4])
s2 = list(set(s2))
s2.sort()
ans = s2[K-1]
print(ans)