output_description: Print the K-th lexicographically smallest substring of K.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      s
      K
attempt: s = input()
  k = int(input())
  st = set()
  for i in range(len(s)):
      for j in range(min(k, len(s)-i+1))
          st.add(s[i:i+j])
  for _ in range(k-1):
      m = min(st)
      st = [x for x in st if x != m]
  print(min(st))
  
problem_description: 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}.
samples: [{"input": "aba\n    4", "output": "b\n    \n\ns has five substrings: `a`, `b`, `ab`, `ba` and `aba`. Among them, we should\nprint the fourth smallest one, `b`. Note that we do not count `a` twice.\n\n* * *"}, {"input": "atcoderandatcodeer\n    5", "output": "andat\n    \n\n* * *"}, {"input": "z\n    1", "output": "z"}]