#define TEST_MODE true
 
#define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES
 
#include "bits/stdc++.h"
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define rep2(i, a, b) for (int i = (a); i < (int)(b); ++i)
#define rrep(i, n) for (int i = (n)-1; i >= 0; --i)
#define rrep2(i, a, b) for (int i = (a)-1; i >= (int)b; --i)
#define range(i, a, b, c) for (int i = a;		\
                               c > 0 ? i < b : i > b;	\
                               i += c)
#define chmax(a, b) (a = (a) < (b) ? (b) : (a))
#define chmin(a, b) (a = (a) > (b) ? (b) : (a))
using ll = long long;
using ull = unsigned long long;
using ld = long double;
#define all(a) a.begin(), a.end()
#define ifnot(a) if (not(a))
#define int long long
 
#if TEST_MODE == true
const bool test = true;
#define dump(x) cerr << #x << " = " << (x) << " "
#define dumpl(x) cerr << #x << " = " << (x) << " " << endl;
#else
const bool test = false;
#define dump(x) 
#define dumpl(x) 
#endif
 
int dx[] = {1, 0, -1, 0};
int dy[] = {0, 1, 0, -1};
const int INF = (int)1 << 60;
const ll INFL = (ll)1 << 60;
ll mod_n = (int)1e9 + 7;
const double eps = 1e-10;
typedef long double Real;
// return -1, 0, 1
int sgn(const Real &r) { return (r > eps) - (r < -eps); }
int sgn(const Real &a, const Real &b) { return sgn(a - b); }
 
//.....................
const int MAX = (int)2e5 + 5;
 
vector<string> split(const string &str, char sep)
{
  vector<string> v;
  stringstream ss(str);
  string buffer;
  while (getline(ss, buffer, sep))
    {
      v.push_back(buffer);
    }
  return v;
}
 
template <class InputIterator>
int sum(InputIterator begin, InputIterator end)
{
  return accumulate(begin, end, 0ll);
}
 
template<typename T>
T gcd(T a, T b)
{
  T c;
  while (a != 0) {
    c = a; a = b%a;  b = c;
  }
  return b;
}

int pow(int a, int b) {
  int res = 1;
  rep(i, b) res *= a;
  return res;
}
 
class Solver {
public:
  string s;
  int K;

  void solve() {
    cin >> s >> K;

    set<string> set1;
    rep(i, s.size()) {
      string s1 = "";
      rep2(j, i, s.size()) {
        s1 += s[j];
        if ((int)set1.size() <= K) {
          set1.insert(s1);
        }
        else if (s1 < *set1.rbegin()) {
          set1.erase(*set1.rbegin());
          set1.insert(s1);
        }
      }
    }
    for (auto s1 : set1) {
      // dumpl(s1);
    }
    // cout << *set1.rbegin() << endl;
    {
      int i = 0;
      for (auto s1 : set1) {
        i += 1;
        // dumpl(s1);
        if (i == K) { cout << s1 << endl; break;}
      }
    }
  }
};
 
signed main()
{
  srand(time(NULL));
  cout << fixed << setprecision(20);
  auto ptr = new Solver();
  ptr->solve();
  delete ptr;
  // while (true) {    
  //     auto ptr = new Solver();
  //     ptr->solve();
  //     delete ptr;
  // }
  return 0;
}
 
// class Mycin {
// 	bool flag = true;
// public:
// 	Mycin& operator >> (int& a) {flag = scanf("%lld", &a) != EOF; return *this;}
// 	Mycin& operator >> (char& a) {flag = scanf("%c", &a) != EOF; return *this;}
// 	Mycin& operator >> (string& s) {flag = (bool)(cin >> s); return *this;}
// 	operator bool() {return flag;}
// } mycin;
 
// class Mycout {
// public:
// 	Mycout& operator << (const int& a) {printf("%lld", a); return *this;}
// 	Mycout& operator << (const char c) {printf("%c", c); return *this;}
// 	Mycout& operator << (const string& s) {printf("%s", s.c_str()); return *this;}
// } mycout;
 
// #define cin mycin
// #define cout mycout
// #define endl '\n'


Measured memory_bytes: 384