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)

def main():
    s = input()
    k = int(input())
    l = set()
    ls = len(s)
    clist = sorted(list(set(list(s))))
    ladd = l.add
    for i in clist:
        x = []
        for j in range(ls):
            if i == s[j]:
                x.append(j)
        ladd(i)
        for j in range(len(x)):
            idx = x[j]
            xxx = s[idx]
            c = 0
            for t in range(idx+1,ls):
                xxx = xxx +s[t]
                ladd(xxx)
                c += 1
                if c > k:
                    break
        if len(l) >= k:
            l = list(l)
            print(sorted(l)[k-1])
            break
    
if __name__ == '__main__':
    main()