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: Time Limit Exceeded)

s=input()
N=int(input())
#s="atcoderandatcodeer"
#N=5
D={}

if(len(s)<=10):
    for i in range(1,len(s)+1):
        for j in range(len(s)):
            if D.get(s[j:j+i],None) == None:
                D[s[j:j+i]]=1
    D=sorted(D)
    print(D[N-1])
    exit()
else:
    t=sorted(s)
    i=0
    k=0
    for c in t:
        if(c!=k):
            i=0
            if(len(D)>=5):
                break
        k=c
        while(i<len(s)):
            if(s[i]==c):
                break
            i+=1
        j=i+1
        while(j<=len(s)):
            D[s[i:j]]=1
            j+=1
        i+=1
    
    D=sorted(D)
    print(D[N-1])
    exit()