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>

char ss[25000][6];

int compare(const void *arg1, const void *arg2){
	return(strcmp(ss[*(int*)arg1], ss[*(int*)arg2]));
}

int main(){
	char s[5001];
	int k,g=0,i,j,c=0,l;
	scanf("%s%d",s,&k);
	while(s[g]!=0)g++;
	int num[25000];
	for(i=0;i<25000;i++)num[i]=i;
	for(i=0;i<g;i++){
		for(j=0;j<5&&i+j<g;j++){
			for(l=0;l<=j;l++)ss[c][l]=s[i+l];
			ss[c][5]='\0';
			c++;
		}
	}
//	for(i=0;i<c;i++)printf("%s\n",ss[i]);
	qsort(num,c,sizeof(int),compare);
/*	for(i=0;i<c;i++){
		for(j=0;j<i;j++){
			if(strcmp(ss[num[i]],ss[num[j]])<0){
*//*				char tmp[7]={0};
				strcpy(tmp,ss[i]);
				for(l=0;l<5;l++)ss[i][l]='\0';
				strcpy(ss[i],ss[j]);
				for(l=0;l<5;l++)ss[j][l]='\0';
				strcpy(ss[j],tmp);
*//*			
			int tmp=num[i];
			num[i]=num[j];
			num[j]=tmp;
			}
		}
	}*/
//	for(i=0;i<c;i++)printf("%s\n",ss[i]);
	int count=0;
	for(i=0;i<c;i++){
		if(count==k-1){
			printf("%s\n",ss[num[i]]);
			return 0;
		}
		if(strcmp(ss[num[i]],ss[num[i+1]])<0)count++;
	}
}