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>
#define REP(i, n) for(ll i = 0; i < n; i++)
#define REPR(i, n) for(ll i = n; i >= 0; i--)
#define FOR(i, m, n) for (ll i = m; i < n; i++)
#define INF LLONG_MAX
#define ALL(v) v.begin(), v.end()
using namespace std;
typedef long long ll;
typedef vector<ll> vll;
typedef vector<vll> vvll;


int main()
{
    string s; cin >> s;
    ll k; cin >> k;
    set<string> st;
    for (int size = 1; size <= s.size(); size++)
    {
        for (int i = 0; i < s.size() - size + 1; i++) {
            st.insert(s.substr(i, size));
        }
    }

    // for (auto elem : st) cout << elem << " "; cout << endl;

    vector<string> ks(st.begin(), st.end());
    // for (auto elem : ks) cout << elem << " "; cout << endl;

    cout << ks[k-1] << endl;
    return 0;
}