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: Accepted)

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <queue>
#include <functional>
#include <map>
#include <set>
#include <sys/timeb.h>
#include <fstream>
#include <random>
#include <regex>
#include <chrono>
#include <bitset>
#include <cassert>
#include <iomanip>
#include <limits>

using namespace std;

#define DEBUG_

#define repr(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define rep(i,n) repr(i,0,n)
#define reprrev(i,a,b) for(int i=(int)(b)-1;i>=(int)(a);i--)
#define reprev(i,n) reprrev(i,0,n)
#define repi(itr,ds) for(auto itr=ds.begin();itr!=ds.end();itr++)
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
#define mp make_pair
#define mt make_tuple
#define INF 1050000000
#define INFR INT_MAX
#define INFL (long long)(4e18)
#define INFLR LLONG_MAX
#define EPS (1e-10)
#define MOD 1000000007
#define PI 3.141592653589793238
#define RMAX 4294967295


#ifdef DEBUG_
#define DEB
#else
#define DEB if(false)
#endif

#ifdef DEBUG_
#define dump(...) DUMPOUT<<"  "; \
dump_func(string(#__VA_ARGS__) + ":", "[" + to_string(__LINE__) + ":" + __FUNCTION__ + "]"); \
DUMPOUT<<"    "; \
dump_func(__VA_ARGS__)
#else
#define dump(...)
#endif

typedef long long ll;
typedef pair<int, int> Pi;
typedef pair<ll, ll> Pll;
typedef pair<double, double> Pd;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<bool> vb;
typedef vector<char> vc;
typedef vector<string> vs;
typedef vector<double> vd;
typedef vector<Pi> vPi;
typedef vector<Pll> vPll;
typedef vector<Pd>vPd;
typedef vector<vector<int> > vvi;
typedef vector<vector<bool> > vvb;
typedef vector<vector<ll> > vvll;
typedef vector<vector<char> > vvc;
typedef vector<vector<string> > vvs;
typedef vector<vector<double> > vvd;
typedef vector<vector<Pi> > vvPi;
typedef vector<vector<vector<int> > > vvvi;
typedef vector<vector<vector<ll> > > vvvll;
typedef priority_queue<int, vector<int>, greater<int> > pqli;
typedef priority_queue<ll, vector<ll>, greater<ll> > pqlll;
typedef priority_queue<Pi, vector<Pi>, greater<Pi> > pqlP;
template <class T>
using vec = vector<T>;
template<class T>
using pql = priority_queue<T, vector<T>, greater<T>>;
string debug_show(Pi a) {
	return "(" + to_string(a.first) + "," + to_string(a.second) + ")";
}

#define DUMPOUT cerr

void dump_func() {
	DUMPOUT << endl;
}
template <class Head, class... Tail>
void dump_func(Head&& head, Tail&&... tail) {
	DUMPOUT << head;
	if (sizeof...(Tail) <= 1) {
		DUMPOUT << " ";
	}
	else {
		DUMPOUT << ", ";
	}
	dump_func(std::move(tail)...);
}

// vector
template<typename T>
istream& operator >> (istream& is, vector<T>& vec) {
	for (T& x : vec) is >> x;
	return is;
}
// pair
template<typename T, typename U>
ostream& operator << (ostream& os, pair<T, U>& pair_var) {
	os << "(" << pair_var.first << "," << pair_var.second << ")";
	return os;
}
// pair
template<typename T, typename U>
ostream& operator << (ostream& os, const pair<T, U>& pair_var) {
	os << "(" << pair_var.first << "," << pair_var.second << ")";
	return os;
}
// vector
template<typename T>
ostream& operator << (ostream& os, const vector<T>& vec) {
	os << "{";
	for (int i = 0; i < vec.size(); i++) {
		os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
	}
	os << "}";
	return os;
}
// map
template<typename T, typename U>
ostream& operator << (ostream& os, map<T, U>& map_var) {
	os << "{";
	repi(itr, map_var) {
		os << "(" << itr->first << "," << itr->second << ")";
		itr++;
		if (itr == map_var.end()) os << ", ";
		itr--;
	}
	os << "}";
	return os;
}
// set
template<typename T>
ostream& operator << (ostream& os, set<T>& set_var) {
	os << "{";
	repi(itr, set_var) {
		os << "  " << *itr << endl;
		itr++;
		if (itr == set_var.end()) os << ", ";
		itr--;
	}
	os << "}";
	return os;
}
string YN(bool y, int id = 0) { if (id)cout << id; return (y ? "YES" : "NO"); }
string yn(bool y, int id = 0) { if (id)cout << id; return (y ? "Yes" : "No"); }
string ON(bool y, int id = 0) { if (id)cout << id; return (y ? "OK" : "NG"); }

int dir4[4][2] = { { 0,-1 },{ -1,0 },{ 1,0 },{ 0,1 } };
int dir8[8][2] = { { -1,-1 },{ 0,-1 },{ 1,-1 },{ -1,0 },{ 1,0 },{ -1,1 },{ 0,1 },{ 1,1 } };

// [a,b)
int irand(int a, int b) {
	static mt19937 Rand(static_cast<unsigned int>(time(nullptr)));
	uniform_int_distribution<int> dist(a, b - 1);
	return dist(Rand);
}

// [a,b)
double drand(int a, int b) {
	static mt19937 Rand(static_cast<unsigned int>(time(nullptr)));
	uniform_real_distribution<double> dist(a, b);
	return dist(Rand);
}
struct Edge {
	int from, to, cost;
};
using Graph = vec<vec<Edge>>;

//======================================================



int main() {
	string s;
	int K;
	cin >> s >> K;
	pql<string> Q;
	vi num(26, 0);

	reprev(i, s.size()) {
		num[s[i] - 'a'] = s.size() - i;
	}

	int c = 0;
	int cnt = 0;
	rep(i, 26) {
		cnt += num[i];
		if (cnt >= K) {
			c = i;
			break;
		}
	}

	set<string> exist;

	rep(i, s.size()) {
		if (s[i] - 'a' > c)continue;
		repr(j, 1, min(K + 1, (int)s.size() - i + 1)) {
			string t = s.substr(i, j);
			if (exist.find(t) == exist.end()) {
				exist.insert(t);
				Q.push(t);
			}
		}
	}

	rep(i, K - 1) {
		Q.pop();
	}

	cout << Q.top() << endl;

	return 0;
}