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)

/**
  *    author:  NEZZ
 **/
 #include <bits/stdc++.h>
 #define rep(i,a,b) for (int i=a;i<b;i++)
 #define rep2(i,a,b) for (int i=a;i>=b;i--)
 #define mod 1000000007
 #define INF  0x3f3f3f3f3f3f3f3f
 #define f first
 #define s second
 #define endl '\n'
 #define ll long long
 #define ii pair <int,int>
 #define pll pair <ll,ll>
 #define vi vector <int>
 #define vl vector <ll>
 #define vvi vector < vi >
 #define vii vector < ii >
 #define vvii vector < vii>
 #define vll vector < ll >
 #define vb  vector <bool>
 #define pb push_back
 #define mk make_pair
 #define sz(a) a.size()
 #define all(a) a.begin(),a.end()
 #define rall(a) a.rbegin(),a.rend()
 #define input freopen("input","r",stdin)
 #define output freopen("output","w",stdout)
 #define fastIO ios_base::sync_with_stdio (0),cin.tie(0)
 #define debug input,output

 using namespace std;
 void affiche (ii t[],int n){ rep (i,0,n) cout<<t[i].s<<" ";cout<<endl;}
 void afficheV (vll t){rep (i,0,sz(t))cout<<t[i]<<" ";cout<<endl;}
int const MAXN=1e5+7;
int main(){
 fastIO;
 string s;
 cin>>s;
 int n;
 cin>>n;
  set <string> v;

 rep(i,0,sz(s)){
   rep(j,i,sz(s)){
     v.insert(s.substr(i,j-i+1));
   }
 }
   set <string>::iterator it=v.begin();
       rep(i,0,n-1)
       it++;
  cout<<*it;

}