#include <bits/stdc++.h>
using namespace std;
vector<vector<int> > c;
vector<int> sort_cyclic_shifts(string const& s) {
  int n = s.size();
  const int alphabet = 256;
  vector<int> p(n), cnt(alphabet, 0);
  c.clear();
  c.emplace_back();
  c[0].resize(n);
  for (int i = 0; i < n; i++) cnt[s[i]]++;
  for (int i = 1; i < alphabet; i++) cnt[i] += cnt[i - 1];
  for (int i = 0; i < n; i++) p[--cnt[s[i]]] = i;
  c[0][p[0]] = 0;
  int classes = 1;
  for (int i = 1; i < n; i++) {
    if (s[p[i]] != s[p[i - 1]]) classes++;
    c[0][p[i]] = classes - 1;
  }
  vector<int> pn(n), cn(n);
  cnt.resize(n);
  for (int h = 0; (1 << h) < n; ++h) {
    for (int i = 0; i < n; i++) {
      pn[i] = p[i] - (1 << h);
      if (pn[i] < 0) pn[i] += n;
    }
    fill(cnt.begin(), cnt.end(), 0);
    for (int i = 0; i < n; i++) cnt[c[h][pn[i]]]++;
    for (int i = 1; i < classes; i++) cnt[i] += cnt[i - 1];
    for (int i = n - 1; i >= 0; i--) p[--cnt[c[h][pn[i]]]] = pn[i];
    cn[p[0]] = 0;
    classes = 1;
    for (int i = 1; i < n; i++) {
      pair<int, int> cur = {c[h][p[i]], c[h][(p[i] + (1 << h)) % n]};
      pair<int, int> prev = {c[h][p[i - 1]], c[h][(p[i - 1] + (1 << h)) % n]};
      if (cur != prev) ++classes;
      cn[p[i]] = classes - 1;
    }
    c.push_back(cn);
  }
  return p;
}
vector<int> suffix_array_construction(string s) {
  s += "!";
  vector<int> sorted_shifts = sort_cyclic_shifts(s);
  sorted_shifts.erase(sorted_shifts.begin());
  return sorted_shifts;
}
int compare(int i, int j, int n, int k) {
  pair<int, int> a = {c[k][i], c[k][(i + 1 - (1 << k)) % n]};
  pair<int, int> b = {c[k][j], c[k][(j + 1 - (1 << k)) % n]};
  return a == b ? 0 : a < b ? -1 : 1;
}
int lcp(int i, int j) {
  int log_n = c.size() - 1;
  int ans = 0;
  for (int k = log_n; k >= 0; k--) {
    if (c[k][i] == c[k][j]) {
      ans += 1 << k;
      i += 1 << k;
      j += 1 << k;
    }
  }
  return ans;
}
vector<int> lcp_construction(string const& s, vector<int> const& p) {
  int n = s.size();
  vector<int> rank(n, 0);
  for (int i = 0; i < n; i++) rank[p[i]] = i;
  int k = 0;
  vector<int> lcp(n - 1, 0);
  for (int i = 0; i < n; i++) {
    if (rank[i] == n - 1) {
      k = 0;
      continue;
    }
    int j = p[rank[i] + 1];
    while (i + k < n && j + k < n && s[i + k] == s[j + k]) k++;
    lcp[rank[i]] = k;
    if (k) k--;
  }
  return lcp;
}
int main() {
  string s;
  cin >> s;
  int K;
  cin >> K;
  int n = s.size();
  vector<int> p = suffix_array_construction(s);
  vector<int> lcp = lcp_construction(s, p);
  for (int i = 0; i < p.size(); i++) {
    for (int j = 1 + (i > 0 ? lcp[i - 1] : 0); j <= n - p[i]; j++) {
      if (K == 1) {
        cout << s.substr(p[i], j) << endl;
        return 0;
      }
      K--;
      int mn = j;
      for (int x = i + 1; x < p.size(); x++) {
        mn = min(mn, lcp[x - 1]);
        if (mn < j) break;
        if (K == 1) {
          cout << s.substr(p[x], j) << endl;
          return 0;
        }
        K--;
      }
    }
  }
  cout << "No such line." << endl;
  return 0;
}
