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<cstdio>
#include<algorithm>
#include<queue>
#include<iostream>
#include<cmath>
#include<vector>
#include<set>
#include<stack>
#include<map>
#include<cstring>
#define eps 1e-8
#define pi acos(-1)
#define mp make_pair
#define PH push
#define pb push_back
#define PII pair<ll,ll>
#define VI vector<int>
#define fi first
#define se second
#define N(a,b) a=N(a,b)
#define Min(a,b) a=min(a,b)
#define sqr(x) ((x)*(x))
#define CL(a) memset(a,0,sizeof a)
#define fr(i,n) for(int i=1;i<=n;i++)
#define FR(i,S,n) for(int i=S;i<=n;i++)
#define dr(i,n) for(int i=n;i;i--)
#define DR(i,n,S) for(int i=n;i>=S;i--)
#define SZ(x) ((int)(x).size())
#define all(n) n.begin(),n.end()
#define rd(x) scanf("%d",&x)
#define pr(x) printf("%d",x)
#define prn(x) printf("%d\n",x)
typedef long long ll;
typedef unsigned long long ull;
ll expow(ll a,ll b,ll p) {ll v=1; for (; b; b>>=1,a=a*a%p)if (b&1)v=v*a%p; return v;}
ll inv(ll a,ll p) {return expow(a,p-2,p);}
using namespace std;
const int N=1000+10;
set<string>q;
int main(){
    string s;
    cin>>s;
    int ss=s.size();
    for(int j=0;j<ss;j++){
        for(int k=1;k+j<=ss;k++){
            q.insert(s.substr(j,k));
        }
    }
    int sq=q.size();
    int n;
    cin>>n;
    auto pos=q.begin();
    for(int i=0;i<n-1;i++)pos++;
    cout<<*pos<<endl;
    return 0;
}