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 C++14 (GCC 5.4.1) (judge verdict: Time Limit Exceeded)

#include <bits/stdc++.h>
using namespace std;
const long long N = 1e6;
int d()
{
    int ret;
    scanf("%d", &ret);
    return ret;
}
long long lld()
{
    long long ret;
    scanf("%lld", &ret);
    return ret;
}
int main()
{
    int k;
    string s;
    cin>>s>>k;
    set<string>se;
    for(int i=0; i<s.length(); i++)
    {
        string s1="";
        s1+=s[i];
        se.insert(s1);
        for(int j=i+1; j<s.length(); j++)
        {
            s1+=s[j];
            se.insert(s1);
        }
    }
    int i=1;
    for(auto v:se)
    {
        if(i==k)
        {
            cout<<v<<endl;
        }
        i++;
    }
}