#腾讯笔试#8.23  腾讯笔试第二题为什么回溯一个都过不了

import sys
class Solution:
    def subset(self,s,n):
        self.res = []
        partial = ""
        self.used = [False] * len(s)
        self.backtracking(partial,s)
        self.res.sort()
        return self.res[n-1]
    def backtracking(self,partial,s):
        if partial:
            self.res.append(partial)
        if len(partial) == len(s):
            return
        else:
            for i in range(len(s)):
                if not self.used[i]:
                    if not partial or ord(s[i]) >= ord(partial[-1]):
                        if i > 0 and s[i] == s[i-1] and not self.used[i - 1]:
                            continue
                        self.used[i] = True
                        partial+=s[i]
                        self.backtracking(partial,s)
                        partial = partial[:-1]
                        self.used[i] = False
a = input().split()
s = a[0]
n = int(input().split()[0])
print(str(Solution().subset(s,n)))