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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define ll long long
#define rep(i,l,r)for(ll i=(l);i<(r);i++)
int ups(const void*a, const void*b){return strcmp((char*)a,(char*)b);}

char s[5010];
char ss[30000][8];
int main(){
	int k;
	scanf("%s%d",s,&k);
	int n=strlen(s);
	int cnt=0;
	rep(i,0,n)rep(j,0,5){
		rep(kk,0,j+1)ss[cnt][kk]=s[i+kk];
		cnt++;
	}
	qsort(ss,cnt,8,ups);
		
	if(k==1){
		puts(ss[0]);
		return 0;
	}
	int num=1,pre=0;
	rep(i,1,cnt){
		if(strcmp(ss[pre],ss[i])){
			num++;
			pre=i;
			if(num==k){
				puts(ss[i]);
				return 0;
			}
		}
	}
}