#define _CRT_SECURE_NO_WARNINGS
#include "bits/stdc++.h"
#include <random>
#include <unordered_map>
#include <unordered_set>

using namespace std;

//呪文
#define DUMPOUT cerr
#define dump(...) DUMPOUT<<"  ";DUMPOUT<<#__VA_ARGS__<<" :["<<__LINE__<<":"<<__FUNCTION__<<"]"<<endl;DUMPOUT<<"    ";dump_func(__VA_ARGS__)

typedef unsigned uint; typedef long long ll; typedef unsigned long long ull; typedef pair<int, int> pii; typedef pair<ll, ll> pll; typedef pair<double, double> pdd; typedef pair<string, string> pss;
template <typename _KTy, typename _Ty> ostream& operator << (ostream& ostr, const pair<_KTy, _Ty>& m) { ostr << "{" << m.first << ", " << m.second << "}"; return ostr; }
template <typename _KTy, typename _Ty> ostream& operator << (ostream& ostr, const map<_KTy, _Ty>& m) { if (m.empty()) { ostr << "{ }"; return ostr; } ostr << "{" << *m.begin(); for (auto itr = ++m.begin(); itr != m.end(); itr++) { ostr << ", " << *itr; } ostr << "}"; return ostr; }
template <typename _Ty> ostream& operator << (ostream& ostr, const vector<_Ty>& v) { if (v.empty()) { ostr << "{ }"; return ostr; } ostr << "{" << v.front(); for (auto itr = ++v.begin(); itr != v.end(); itr++) { ostr << ", " << *itr; }	ostr << "}"; return ostr; }
template <typename _Ty> ostream& operator << (ostream& ostr, const set<_Ty>& s) { if (s.empty()) { ostr << "{ }"; return ostr; } ostr << "{" << *(s.begin()); for (auto itr = ++s.begin(); itr != s.end(); itr++) { ostr << ", " << *itr; }	ostr << "}"; return ostr; }
template <typename _Ty> ostream& operator << (ostream& ostr, const stack<_Ty>& s) { if (s.empty()) { ostr << "{ }"; return ostr; } stack<_Ty> t(s); ostr << "{" << t.top(); t.pop(); while (!t.empty()) { ostr << ", " << t.top(); t.pop(); } ostr << "}";	return ostr; }
template <typename _Ty> ostream& operator << (ostream& ostr, const list<_Ty>& l) { if (l.empty()) { ostr << "{ }"; return ostr; } ostr << "{" << l.front(); for (auto itr = ++l.begin(); itr != l.end(); ++itr) { ostr << ", " << *itr; } ostr << "}"; return ostr; }
template <typename _KTy, typename _Ty> istream& operator >> (istream& istr, pair<_KTy, _Ty>& m) { istr >> m.first >> m.second; return istr; }
template <typename _Ty> istream& operator >> (istream& istr, vector<_Ty>& v) { for (size_t i = 0; i < v.size(); i++) istr >> v[i]; return istr; }
namespace aux { // print tuple
	template<typename Ty, unsigned N, unsigned L> struct tp { static void print(ostream& os, const Ty& v) { os << get<N>(v) << ", "; tp<Ty, N + 1, L>::print(os, v); } };
	template<typename Ty, unsigned N> struct tp<Ty, N, N> { static void print(ostream& os, const Ty& value) { os << get<N>(value); } };
}
template<typename... Tys> ostream& operator<<(ostream& os, const tuple<Tys...>& t) { os << "{"; aux::tp<tuple<Tys...>, 0, sizeof...(Tys)-1>::print(os, t); os << "}"; return os; }

template<typename A, size_t N, typename T> inline void Fill(A(&array)[N], const T &val) { std::fill((T*)array, (T*)(array + N), val); }

void dump_func() { DUMPOUT << endl; }
template <class Head, class... Tail> void dump_func(Head&& head, Tail&&... tail) { DUMPOUT << head; if (sizeof...(Tail) == 0) { DUMPOUT << " "; } else { DUMPOUT << ", "; } dump_func(std::move(tail)...); }

#define PI 3.14159265358979323846
#define EPS 1e-11
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define all(x) (x).begin(), (x).end()
#define SZ(x) ((int)(x).size())
#define fake false



class DisjointSet {
private:
	vector<unsigned> par;
	vector<unsigned> rank;
	vector<unsigned> sz;	// 要素の個数
	size_t set_sz;			// 集合の個数

public:

	DisjointSet() : set_sz(0) { }

	DisjointSet(size_t n) :
		par(n), rank(n, 0), sz(n, 1), set_sz(n) {
		for (size_t i = 0; i < n; i++) par[i] = i;
	}

	size_t set_size() { return set_sz; }
	size_t size(size_t v) { return sz[find(v)]; }

	void makeset() {
		par.push_back(par.size());
		rank.push_back(0);
		sz.push_back(1);
		set_sz++;
	}
	void makeset(size_t n) {
		for (size_t i = 0; i < n; i++) makeset();
	}

	size_t find(size_t x) {
		if (par[x] == x)
			return x;
		else
			return par[x] = find(par[x]);
	}

	void unite(size_t x, size_t y) {
		x = find(x);
		y = find(y);
		if (x == y) return;

		if (rank[x] < rank[y])
			par[x] = y;
		else {
			par[y] = x;
			if (rank[x] == rank[y]) rank[x]++;
		}
		sz[x] += sz[y];
		sz[y] = sz[x];
		set_sz--;
	}

	bool same(size_t x, size_t y) {
		return find(x) == find(y);
	}

	friend ostream& operator<<(ostream& ostr, const DisjointSet& ds) {
		ostr << endl << "parent : " << ds.par << endl << "rank : " << ds.rank << endl << "size : " << ds.sz << endl << "size of set : " << ds.set_sz << endl;
		return ostr;
	}
};


int main() {

	cin.tie(0);
	ios::sync_with_stdio(false);

	int K;
	string s;
	cin >> s >> K;
	
	// 辞書順最小は底にたまる
	priority_queue<string> pq;
	set<string> st;
	
	for (int i = 0; i < s.size(); i++) {
		for (int j = i + 1; j <= s.size(); j++) {
			string sub = s.substr(i, j - i);
			if (st.size() >= 5 && *(--st.end()) < sub) break;
			st.insert(sub);
			if (st.size() > 5) st.erase(*(--st.end()));
		}
	}
	
	auto itr = st.begin();
	for (int i = 1; i < K; i++) {
		itr++;
	}
	cout << *itr << endl;
	return 0;
}


Measured memory_bytes: 256