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: Runtime Error)

#include<stdio.h>
#include<string.h>
#include<math.h>
#include<stdlib.h>
#include<limits.h>

#define rep(i,begin,end) for(int i=begin; i<end; i++)
#define lld long long int

int cmpAscStrings(const void* a, const void* b){
    return strcmp(*(char**)a, *(char**)b);
}

void initStrArray(char** p, int n, int len){
    for(int i=0; i<n; i++){
        *(p + i) = (char*)malloc(sizeof(char) * (len + 1));
    }
}

int main(){
    char s[5001];
    int k, len;
    scanf("%s", s);
    scanf("%d", &k);
    len = strlen(s);
    char* w[3000];
    initStrArray(w, 3000, 11);
    int x=0;
    rep(i, 0, len){
        char buf[10];
        rep(j, 0, 6){
            buf[j] = s[i+j];
            buf[j+1]  = '\0';
            strcpy(w[x], buf);
            x++;
            if(s[i+j] == '\0'){
                break;
            }
        }
    }
    qsort(w, x, sizeof(char*), cmpAscStrings);
    int q=1, id=0;
    while(q<k){
        id++;
        if(strcmp(w[id-1], w[id]) != 0){
            q++;
        }
    }
    printf("%s", w[id]);

    return 0;
}