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: Accepted)

#include <bits/stdc++.h>
using namespace std;

typedef pair<int,int> P;
#define ll long long
#define ld long double
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define PI 3.14159265358979323846
#define sz(x) ((int)(x).size())
#define chmin(x,y) x = min(x,y)
#define chmax(x,y) x = max(x,y)

const int INF = 1e9;
const int MOD = 1e9 + 7;
const ll LINF = 1e18;
int main()
{
    string s;
    cin >> s;
    int k;
    cin >> k;
    vector<string> vc;
    string sub;
    for (int i = 1; i < k+1; i++)
    {
        for (int j = 0; j+i-1<s.length(); j++)
        {
            sub = s.substr(j,i);
            vc.push_back(sub);
        }
    }
    sort(vc.begin(),vc.end());
    unique(vc.begin(),vc.end());
    
    cout << vc[k-1] << endl;
    
    
}