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 PyPy3 (7.3.0) (judge verdict: Accepted)

def suuji(a):
  ans=0
  for i in range(len(a)):
    ans+=(ord(a[i])-96)*(27**(5-i))
  return ans
def alpha(a):
  ans=''
  while a>0:
    b = chr((a%27)+96)
    if b!='`':
      ans+=chr((a%27)+96)
    a//=27
  return ans[::-1]
g = []
s=input()
k=int(input())
for i in range(len(s)):
  for j in range(1,min(6,len(s)+1)):
    if i+j>len(s):
      break
    g.append(suuji(s[i:j+i]))
g = sorted(set(g))
print(alpha(g[k-1]))