Problem p03353 (C++)

Buggy submission (status: Time Limit Exceeded):
#include "bits/stdc++.h"
#define _USE_MATH_DEFINES
#include <math.h>

using namespace std;

using LL = long long;
using VLL = std::vector<LL>;
using VVLL = std::vector<VLL>;
using VVVLL = std::vector<VVLL>;
using LD = long double;
using VLD = std::vector<LD>;
using VVLD = std::vector<VLD>;
using VVVLD = std::vector<VVLD>;
using BL = bool;
using VBL = std::vector<BL>;
using VVBL = std::vector<VBL>;
using VVVBL = std::vector<VVBL>;

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> void bye(T a) {
  cout << a << '\n';
  exit(0);
}

// template<class T=LL> inline vector<T> cinv(LL N){ vector<T> v(N); REP(i,
// N)cin>>v[i]; return move(v);} template<class T=LL> inline vector<T> cinv(LL
// N){ vector<T> v(N); REP(i, N)cin>>v[i]; return move(v);}
#define REP(i, n) for (LL(i) = 0; (i) < (n); (i)++)
#define REPM(i, n, m) for (LL(i) = m; (i) < (n); (i)++)
#define ALL(v) v.begin(), v.end()
#define PRINTLN(v)                                                             \
  {                                                                            \
    LL i = 0;                                                                  \
    for (auto(a) : v) {                                                        \
      cerr << setw(2) << i << " : " << a << endl;                              \
      i++;                                                                     \
    }                                                                          \
  }
// #define PRLLSP(v) for(auto (a): v ) {cerr << a << " ";} cerr << endl

template <class T> void PRINTSP(vector<T> v, size_t w = 3) {
  for (auto(a) : v) {
    cerr << setw(w) << a << " ";
  }
  cerr << endl;
}

struct mll {
  static LL MOD;
  LL val;
  mll(LL v = 0) : val(v % MOD) {
    if (val < 0)
      val += MOD;
  }
  mll operator-() const { return -val; }
  mll operator+(const mll &b) const { return val + b.val; }
  mll operator-(const mll &b) const { return val - b.val; }
  mll operator*(const mll &b) const { return val * b.val; }
  mll operator/(const mll &b) const { return mll(*this) /= b; }
  mll operator+(LL b) const { return *this + mll(b); }
  mll operator-(LL b) const { return *this - mll(b); }
  mll operator*(LL b) const { return *this * mll(b); }
  friend mll operator+(LL a, const mll &b) { return b + a; }
  friend mll operator-(LL a, const mll &b) { return -b + a; }
  friend mll operator*(LL a, const mll &b) { return b * a; }
  mll &operator+=(const mll &b) {
    val = (val + b.val) % MOD;
    return *this;
  }
  mll &operator-=(const mll &b) {
    val = (val + MOD - b.val) % MOD;
    return *this;
  }
  mll &operator*=(const mll &b) {
    val = (val * b.val) % MOD;
    return *this;
  }
  mll &operator/=(const mll &b) {
    LL c = b.val, d = MOD, u = 1, v = 0;
    while (d) {
      LL t = c / d;
      c -= t * d;
      swap(c, d);
      u -= t * v;
      swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0)
      val += MOD;
    return *this;
  }
  mll &operator+=(LL b) { return *this += mll(b); }
  mll &operator-=(LL b) { return *this -= mll(b); }
  mll &operator*=(LL b) { return *this *= mll(b); }
  mll &operator/=(LL b) { return *this /= mll(b); }
  bool operator==(const mll &b) { return val == b.val; }
  bool operator!=(const mll &b) { return val != b.val; }
  bool operator==(LL b) { return *this == mll(b); }
  bool operator!=(LL b) { return *this != mll(b); }
  friend bool operator==(LL a, const mll &b) { return mll(a) == b.val; }
  friend bool operator!=(LL a, const mll &b) { return mll(a) != b.val; }
  friend ostream &operator<<(ostream &os, const mll &a) { return os << a.val; }
  friend istream &operator>>(istream &is, mll &a) { return is >> a.val; }
  static mll Combination(LL a, LL b) {
    chmin(b, a - b);
    if (b < 0)
      return mll(0);
    mll c = 1;
    REP(i, b) c *= a - i;
    REP(i, b) c /= i + 1;
    return c;
  }
  static mll Kumiawase(LL a, LL b) {
    chmin(b, a - b);
    if (b < 0)
      return mll(0);
    return Junretu(a, b) / Kaijou(b);
  }
  static mll Kaijou(LL a) {
    if (a < 0)
      return mll(0);
    mll c = 1;
    for (LL i = 1; i <= a; i++) {
      c *= i;
    }
    return c;
  }
  static mll Junretu(LL a, LL b) {
    if (a < b) {
      return mll(0);
    }
    mll c = 1;
    for (LL i = a; i > a - b; i--) {
      c *= i;
    }

    return c;
  }
  static mll _Junretu(LL a, LL b) {
    if (a < b) {
      return mll(0);
    }
    return Kaijou(a) / (Kaijou(a - b));
  }
  LL get() { return val; }
};
LL mll::MOD = (LL)(1e9 + 7); // 998244353LL;//(LL)(1e9 + 7);
using vmll = std::vector<mll>;
using vvmll = std::vector<vmll>;
using vvvmll = std::vector<vvmll>;
using vvvvmll = std::vector<vvvmll>;

// a^n mod を計算する
LL modpow(LL a, LL n, LL mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

template <typename TTT> vector<LL> arg_sort(vector<TTT> A, bool ascend = true) {
  vector<LL> index(A.size());
  iota(index.begin(), index.end(), 0);
  if (ascend) {
    std::sort(index.begin(), index.end(),
              [&A](TTT i1, TTT i2) { return A[i1] < A[i2]; });
  } else {
    std::sort(index.begin(), index.end(),
              [&A](TTT i1, TTT i2) { return A[i1] > A[i2]; });
  }

  return index;
}

template <typename _Iterator, typename _Compare>
LL num_of_ika(_Iterator _first, _Iterator _last, _Compare key) {
  return (LL)(upper_bound(_first, _last, key) - _first);
}
template <typename _Iterator, typename _Compare>
LL num_of_ookii(_Iterator _first, _Iterator _last, _Compare key) {
  return (LL)(_last - upper_bound(_first, _last, key));
}
template <typename _Iterator, typename _Compare>
LL num_of_chisai(_Iterator _first, _Iterator _last, _Compare key) {
  return (LL)(lower_bound(_first, _last, key) - _first);
}
template <typename _Iterator, typename _Compare>
LL num_of_ijou(_Iterator _first, _Iterator _last, _Compare key) {
  return (LL)(_last - lower_bound(_first, _last, key));
}
template <typename _Iterator, typename _Compare>
LL num_of_onaji(_Iterator _first, _Iterator _last, _Compare key) {
  return (LL)(upper_bound(_first, _last, key) -
              lower_bound(_first, _last, key));
}

struct UnionFindTree {
private:
  vector<LL> UFT;

public:
  UnionFindTree(LL N) {
    UFT.resize(N);
    for (LL i = 0; i < N; i++) {
      UFT[i] = -1;
    }
  }

  LL root(LL i) {
    if (UFT[i] < 0) {
      return i;
    } else {
      LL j = root(UFT[i]);
      UFT[i] = j;
      return j;
    }
  }

  bool same(LL i, LL j) { return (root(i) == root(j)); }

  bool unite(LL i, LL j) {
    if (same(i, j)) {
      return false;
    }

    LL root_i = root(i);
    LL root_j = root(j);
    if (root_i != root_j) {
      if (size(root_i) < size(root_j)) {
        swap(root_i, root_j);
      }
      UFT[root_i] += UFT[root_j];
      UFT[root_j] = root_i;
    }
    return true;
  }

  LL size(LL i) { return -UFT[root(i)]; }

  LL get_root_num() {
    set<LL> roots;
    for (LL i = 0; i < (LL)UFT.size(); ++i) {
      roots.insert(root(i));
    }
    return (LL)roots.size();
  }

  map<LL, vector<LL>> get_root_child() {
    map<LL, vector<LL>> a;
    for (LL i = 0; i < (LL)UFT.size(); ++i) {
      LL j = root(i);
      a[j].push_back(i);
    }
    return a;
  }

  void print() {
    for (LL i = 0; i < (LL)UFT.size(); ++i) {
      cerr << root(i) << " ";
    }
    cerr << endl;
  }
};

#define cmax(a, b)                                                             \
  {                                                                            \
    if (a < b) {                                                               \
      a = b;                                                                   \
    }                                                                          \
  }
#define cmin(a, b)                                                             \
  {                                                                            \
    if (a > b) {                                                               \
      a = b;                                                                   \
    }                                                                          \
  }
#define EACH(a, A) for (auto a : A)
// #define cmax(a,b) a = (a>b ? a:b)
// #define cmin(a,b) a = (a<b ? a:b)
// cin,cout高速化のおまじない＋桁数指定
inline VLL cinvll(LL N, LL minus = 0) {
  VLL A(N);
  REP(i, N) {
    cin >> A[i];
    A[i] -= minus;
  }
  return move(A);
}
inline VVLL zerosll(LL H, LL W, LL val = 0) {
  VVLL A(H, VLL(W, val));
  return move(A);
}

#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)

// 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;
}
// vector
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {

  for (LL i = 0; i < (LL)vec.size(); i++) {
    // os << setw(3) <<i <<" : "<< setw(5) << vec[i] << endl;
    os << setw(5) << vec[i];
  }
  os << endl;
  return os;

#if 0    
    os << "{";
    for (LL i = 0; i < (LL)vec.size(); i++) {
        os << vec[i] << (i + 1 == (LL)vec.size() ? "" : ", ");
    }
    os << "}";
    return os;
#endif
}
// map
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var) {
  os << "{";
  repi(itr, map_var) {
    os << *itr;
    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;
    itr++;
    if (itr != set_var.end())
      os << ", ";
    itr--;
  }
  os << "}";

  return os;
}

#define DUMPOUT cerr
#define DUMPCOLOR ("\033[36m")

void dump_func() { DUMPOUT << endl; }
template <class Head, class... Tail>
void dump_func(Head &&head, Tail &&...tail) {
  DUMPOUT << DUMPCOLOR;
  DUMPOUT << head;
  if (sizeof...(Tail) > 0) {
    DUMPOUT << ", ";
  }
  dump_func(std::move(tail)...);
  DUMPOUT << "\033[m";
}
#ifdef DEBUG_
#define DEB
#define dump(...)                                                              \
  DUMPOUT << DUMPCOLOR << "" << string(#__VA_ARGS__) << ": "                   \
          << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]" << endl  \
          << "\033[m"                                                          \
          << "",                                                               \
      dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif

struct Fast {
  Fast() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(std::numeric_limits<double>::max_digits10);
  }
} fast;

#if 0 
LL GCD(LL m, LL n) {

  if (m < n) {
    swap(m, n);
  }

  while (n != 0) {
    LL n_new = m % n;
    m = n;
    n = n_new;
  }

  return m;

}

VLL B;
VVLL T;
LL cnt=0;
void dfs(LL me,LL oya){

  for(LL i=0;i<T[me].size();i++){
    LL ko = T[me][i];
    if(ko==oya){
      continue;
    }
    cnt += min(B[me],B[ko]);
    dfs(ko,me);
  }

}
#endif

bool isPrime(LL N) {
  if (N < 2) {
    return false;
  } else if (N == 2) {
    return true;
  } else if (N % 2 == 0) {
    return false;
  }

  LD sqrtNum = sqrt(N);
  for (LL i = 3; i <= sqrtNum; i += 2) {
    if (N % i == 0) {
      return false;
    }
  }
  return true;
}

#if 0
VLL A;
LL N,M;
// index が条件を満たすかどうか
bool isOK(LL X) {

  LL cnt=0;
  REP(i,N){
    LL hidari=A[i];
    auto it = lower_bound(ALL(A),X-hidari);
    cnt += A.end()-it;
  }
  dump(cnt);
  if(cnt>=M){
    return true;
  }
  else{
    return false;
  }
}
// 汎用的な二分探索のテンプレ
LL binary_search() {
  LL ng = 1e10+1 ;
  LL ok = 0;

  /* ok と ng のどちらが大きいかわからないことを考慮 */
  while (abs(ok - ng) > 1) {
      LL mid = (ok + ng) / 2;

      if (isOK(mid)) ok = mid;
      else ng = mid;
  }
  return ok;
}

map< LL, LL > prime_factor(LL n) {
  map< LL,LL > ret;
  for(LL i = 2; i * i <= n; i++) {
    while(n % i == 0) {
      ret[i]++;
      n /= i;
    }
  }
  if(n != 1) ret[n] = 1;
  return ret;
}

LL GCD(LL m, LL n) { 
  if (m < n) {
    swap(m, n);
  }

  while (n != 0) {
    LL n_new = m % n;
    m = n;
    n = n_new;
  }
  return m;
}
LL LCM(LL x, LL y) { 
  return x / GCD(x, y) * y; 
}

#endif

struct Furui {
  LL n;
  VLL f;
  VLL prime;

  Furui(LL n = 1) : n(n), f(n + 1) {
    f[0] = -1;
    f[1] = -1;
    for (LL i = 2; i <= n; i++) {
      if (f[i]) {
        continue;
      }
      prime.push_back(i);
      for (LL j = i; j <= n; j += i) {
        if (f[j] == 0) {
          f[j] = i;
        }
      }
    }
  }
  bool isPrime(LL x) {
    if (x <= 1) {
      return false;
    }
    return (f[x] == x);
  }
  VLL factorList(LL x) {
    VLL res;
    if (x >= 2 and x <= n) {
      while (x != 1) {
        res.push_back(f[x]);
        x /= f[x];
      }
    }
    return res;
  }
  vector<pair<LL, LL>> factor(LL x) {
    VLL factors = factorList(x);
    vector<pair<LL, LL>> P;
    if (factors.size() == 0) {
      return P;
    }
    P.emplace_back(factors[0], 0);
    for (auto p : factors) {
      if (P.back().first == p) {
        P.back().second++;
      } else {
        P.emplace_back(p, 1);
      }
    }
    return P;
  }
};

template <typename T> struct edge {
  int src, to;
  T cost;
  edge(int to, T cost) : src(-1), to(to), cost(cost) {}
  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}
  edge &operator=(const int &x) {
    to = x;
    return *this;
  }

  operator int() const { return to; }
};
template <typename T> using Edges = vector<edge<T>>;
template <typename T> using WeightedGraph = vector<Edges<T>>;
using UnWeightedGraph = vector<vector<int>>;
template <typename T> using Matrix = vector<vector<T>>;

template <typename T> void warshall_floyd(Matrix<T> &g, T INF) {
  for (int k = 0; k < g.size(); k++) {
    for (int i = 0; i < g.size(); i++) {
      for (int j = 0; j < g.size(); j++) {
        if (g[i][k] == INF || g[k][j] == INF)
          continue;
        g[i][j] = min(g[i][j], g[i][k] + g[k][j]);
      }
    }
  }
}

template <typename T> T kruskal(Edges<T> &edges, LL V) {
  sort(begin(edges), end(edges),
       [](const edge<T> &a, const edge<T> &b) { return (a.cost < b.cost); });
  UnionFindTree tree(V);
  T ret = 0;
  for (auto &e : edges) {
    if (tree.unite(e.src, e.to))
      ret += e.cost;
  }
  return (ret);
}

map<LL, LL> prime_factor(LL n) {
  map<LL, LL> ret;
  for (LL i = 2; i * i <= n; i++) {
    while (n % i == 0) {
      ret[i]++;
      n /= i;
    }
  }
  if (n != 1)
    ret[n] = 1;
  return ret;
}

#if 0
VLL A;
VLL B;
  
bool isOK(LL X) {


  LL cnt=0;
  REP(i,N){
    LL hidari=A[i];
    auto it = lower_bound(ALL(A),X-hidari);
    cnt += A.end()-it;
  }
  dump(cnt);
  if(cnt>=M){
    return true;
  }
  else{
    return false;
  }
}
// 汎用的な二分探索のテンプレ
LL binary_search() {
  LL ng = 0 ;
  LL ok = 100000;

  /* ok と ng のどちらが大きいかわからないことを考慮 */
  while (abs(ok - ng) > 1) {
      LL mid = (ok + ng) / 2;

      if (isOK(mid)) ok = mid;
      else ng = mid;
  }
  return ok;
}

void funcE()
{
  LL H,N;
  cin >>H >> N;
  VLL A;
  VLL B;
  A.resize(N);
  B.resize(N);
  REP(i,N){
    cin >> A[i];
    cin >> B[i];
  }
  dump(H,N);
  dump(A);
  dump(B);
}
#endif
LL powll(LL a, LL b) {
  LL c = 1LL;
  REP(i, b) { c *= a; }
  return c;
}

LL knapsack(vector<LL> v, vector<LL> w, LL W) {
  LL N = (LL)v.size();
  vector<LL> dp(W + 1, 0);
  for (LL i = 0; i < N; ++i) {
    for (LL j = w[i]; j <= W; ++j) {
      dp[j] = max(dp[j], dp[j - w[i]] + v[i]);
    }
  }

  return dp[W];
}

#if 0 

LL H,W;
VVLL M;
VVLL V;
LL sx;
LL sy;
LL gx;
LL gy;

void MeiroDFS(LL y,LL x){

  if(V[y][x]==1){
    return;
  }
  V[y][x]=1;

  if(y>0 and M[y-1][x]==0){
    dfs(y-1,x);
  }
  if(x>0 and M[y][x-1]==0){
    dfs(y,x-1);
  }
  if(y+1<H and M[y+1][x]==0){
    dfs(y+1,x);
  }
  if(x+1<W and M[y][x+1]==0){
    dfs(y,x+1);
  }

}

LL H,W;
VVLL M;
VVLL V;
LL sx;
LL sy;
LL gx;
LL gy;
void MeiroBFS(LL y,LL x){

  queue<tuple<LL,LL,LL>> Q;
  Q.emplace(y,x,0);
  V[y][x]=0;

  LL cnt=0;
  while(not Q.empty()){
    tuple<LL,LL,LL> q = Q.front();
    Q.pop();
    y = get<0>(q);
    x = get<1>(q);
    cnt = get<2>(q);
    if(y>0 and M[y-1][x]==0 and V[y-1][x]==-1){
      V[y-1][x]=cnt+1;
      Q.emplace(y-1,x,cnt+1);
    }
    if(x>0 and M[y][x-1]==0 and V[y][x-1]==-1){
      V[y][x-1]=cnt+1;
      Q.emplace(y,x-1,cnt+1);
    }
    if(y+1<H and M[y+1][x]==0 and V[y+1][x]==-1){
      V[y+1][x]=cnt+1;
      Q.emplace(y+1,x,cnt+1);
    }
    if(x+1<W and M[y][x+1]==0 and V[y][x+1]==-1){
      V[y][x+1]=cnt+1;
      Q.emplace(y,x+1,cnt+1);
    }
  }



}
#endif

template <typename T> vector<T> dijkstra(WeightedGraph<T> &g, int s) {
  const auto INF = numeric_limits<T>::max();
  vector<T> dist(g.size(), INF);

  using Pi = pair<T, int>;
  priority_queue<Pi, vector<Pi>, greater<Pi>> que;
  dist[s] = 0;
  que.emplace(dist[s], s);
  while (!que.empty()) {
    T cost;
    int idx;
    tie(cost, idx) = que.top();
    que.pop();
    if (dist[idx] < cost)
      continue;
    for (auto &e : g[idx]) {
      auto next_cost = cost + e.cost;
      if (dist[e.to] <= next_cost)
        continue;
      dist[e.to] = next_cost;
      que.emplace(dist[e.to], e.to);
    }
  }
  return dist;
}

template <typename T> struct BinaryIndexedTree {
  vector<T> data;

  BinaryIndexedTree(int sz = 10) { data.assign(++sz, 0); }

  T sum(int k) {
    T ret = 0;
    for (++k; k > 0; k -= k & -k)
      ret += data[k];
    return (ret);
  }

  void add(int k, T x) {
    for (++k; k < data.size(); k += k & -k)
      data[k] += x;
  }
  void resize(int sz) { data.assign(++sz, 0); }
};

bool isKaibun2(string S) {
  LL N = (LL)S.length();
  REP(i, (LL)(N / 2)) {
    if (S[i] != S[N - 1 - i]) {
      return false;
    }
  }
  return true;
}

LL Combination(LL a, LL b) {
  chmin(b, a - b);
  if (b < 0)
    return 0;
  LL c = 1;
  REP(i, b) c *= a - i;
  REP(i, b) c /= i + 1;
  return c;
}

struct IntegralImage {
  VVLL I;
  LL H;
  LL W;
  IntegralImage(VVLL M) {
    I = M;
    H = M.size();
    W = M[0].size();
    // check
    bool isOK = true;
    REP(i, H) {
      if (I[i].size() != W) {
        isOK = false;
      }
    }
    if (not isOK) {
      cerr << "shape error :";
      REP(i, H) { cerr << I[i].size() << " "; }
      cerr << endl;
      throw 0;
    }

    REP(i, H) {
      REP(j, W - 1) { I[i][j + 1] += I[i][j]; }
    }
    REP(j, W) {
      REP(i, H - 1) { I[i + 1][j] += I[i][j]; }
    }
  }
  LL get_sum(LL y, LL h, LL x, LL w) {
    if (y < 0 or x < 0 or h < 0 or w < 0 or y + h >= H or x + w >= W) {
      fprintf(stderr, "H=%lld, W=%lld, y=%lld, h=%lld, x=%lld, w=%lld\n", H, W,
              y, h, x, w);
      throw 1;
    }
    LL a1 = I[y + h][x + w];
    LL a2 = 0;
    if (y > 0) {
      a2 = I[y - 1][x + w];
    }
    LL a3 = 0;
    if (x > 0) {
      a3 = I[y + h][x - 1];
    }
    LL a4 = 0;
    if (y > 0 && x > 0) {
      a4 = I[y - 1][x - 1];
    }
    return (a1 + a4 - a2 - a3);
  }
  void print() {
    REP(i, (LL)I.size()) { PRINTSP(I[i]); }
  }
};

bool isKaibun(string S) {
  bool ok = true;
  LL N = (LL)S.length();
  REP(i, N) {
    if (S[i] != S[N - i - 1]) {
      ok = false;
      break;
    }
  }
  return ok;
}

void func() {
  string S;
  LL K;
  cin >> S;
  cin >> K;

  dump(S, K);

  set<string> T;
  LL N = S.length();
  REP(i, N) {
    REPM(j, N, i) {

      string s = S.substr(i, j - i + 1);
      // Xdump(s);
      T.insert(s);
    }
  }

  auto t = T.begin();
  REP(k, K - 1) { t++; }
  bye(*t);

  return;
}
int main() {
  // funcD();
  func();
  return 0;
}


Fixed submission:
#include "bits/stdc++.h"
#define _USE_MATH_DEFINES
#include <math.h>

using namespace std;

using LL = long long;
using VLL = std::vector<LL>;
using VVLL = std::vector<VLL>;
using VVVLL = std::vector<VVLL>;
using LD = long double;
using VLD = std::vector<LD>;
using VVLD = std::vector<VLD>;
using VVVLD = std::vector<VVLD>;
using BL = bool;
using VBL = std::vector<BL>;
using VVBL = std::vector<VBL>;
using VVVBL = std::vector<VVBL>;

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> void bye(T a) {
  cout << a << '\n';
  exit(0);
}

// template<class T=LL> inline vector<T> cinv(LL N){ vector<T> v(N); REP(i,
// N)cin>>v[i]; return move(v);} template<class T=LL> inline vector<T> cinv(LL
// N){ vector<T> v(N); REP(i, N)cin>>v[i]; return move(v);}
#define REP(i, n) for (LL(i) = 0; (i) < (n); (i)++)
#define REPM(i, n, m) for (LL(i) = m; (i) < (n); (i)++)
#define ALL(v) v.begin(), v.end()
#define PRINTLN(v)                                                             \
  {                                                                            \
    LL i = 0;                                                                  \
    for (auto(a) : v) {                                                        \
      cerr << setw(2) << i << " : " << a << endl;                              \
      i++;                                                                     \
    }                                                                          \
  }
// #define PRLLSP(v) for(auto (a): v ) {cerr << a << " ";} cerr << endl

template <class T> void PRINTSP(vector<T> v, size_t w = 3) {
  for (auto(a) : v) {
    cerr << setw(w) << a << " ";
  }
  cerr << endl;
}

struct mll {
  static LL MOD;
  LL val;
  mll(LL v = 0) : val(v % MOD) {
    if (val < 0)
      val += MOD;
  }
  mll operator-() const { return -val; }
  mll operator+(const mll &b) const { return val + b.val; }
  mll operator-(const mll &b) const { return val - b.val; }
  mll operator*(const mll &b) const { return val * b.val; }
  mll operator/(const mll &b) const { return mll(*this) /= b; }
  mll operator+(LL b) const { return *this + mll(b); }
  mll operator-(LL b) const { return *this - mll(b); }
  mll operator*(LL b) const { return *this * mll(b); }
  friend mll operator+(LL a, const mll &b) { return b + a; }
  friend mll operator-(LL a, const mll &b) { return -b + a; }
  friend mll operator*(LL a, const mll &b) { return b * a; }
  mll &operator+=(const mll &b) {
    val = (val + b.val) % MOD;
    return *this;
  }
  mll &operator-=(const mll &b) {
    val = (val + MOD - b.val) % MOD;
    return *this;
  }
  mll &operator*=(const mll &b) {
    val = (val * b.val) % MOD;
    return *this;
  }
  mll &operator/=(const mll &b) {
    LL c = b.val, d = MOD, u = 1, v = 0;
    while (d) {
      LL t = c / d;
      c -= t * d;
      swap(c, d);
      u -= t * v;
      swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0)
      val += MOD;
    return *this;
  }
  mll &operator+=(LL b) { return *this += mll(b); }
  mll &operator-=(LL b) { return *this -= mll(b); }
  mll &operator*=(LL b) { return *this *= mll(b); }
  mll &operator/=(LL b) { return *this /= mll(b); }
  bool operator==(const mll &b) { return val == b.val; }
  bool operator!=(const mll &b) { return val != b.val; }
  bool operator==(LL b) { return *this == mll(b); }
  bool operator!=(LL b) { return *this != mll(b); }
  friend bool operator==(LL a, const mll &b) { return mll(a) == b.val; }
  friend bool operator!=(LL a, const mll &b) { return mll(a) != b.val; }
  friend ostream &operator<<(ostream &os, const mll &a) { return os << a.val; }
  friend istream &operator>>(istream &is, mll &a) { return is >> a.val; }
  static mll Combination(LL a, LL b) {
    chmin(b, a - b);
    if (b < 0)
      return mll(0);
    mll c = 1;
    REP(i, b) c *= a - i;
    REP(i, b) c /= i + 1;
    return c;
  }
  static mll Kumiawase(LL a, LL b) {
    chmin(b, a - b);
    if (b < 0)
      return mll(0);
    return Junretu(a, b) / Kaijou(b);
  }
  static mll Kaijou(LL a) {
    if (a < 0)
      return mll(0);
    mll c = 1;
    for (LL i = 1; i <= a; i++) {
      c *= i;
    }
    return c;
  }
  static mll Junretu(LL a, LL b) {
    if (a < b) {
      return mll(0);
    }
    mll c = 1;
    for (LL i = a; i > a - b; i--) {
      c *= i;
    }

    return c;
  }
  static mll _Junretu(LL a, LL b) {
    if (a < b) {
      return mll(0);
    }
    return Kaijou(a) / (Kaijou(a - b));
  }
  LL get() { return val; }
};
LL mll::MOD = (LL)(1e9 + 7); // 998244353LL;//(LL)(1e9 + 7);
using vmll = std::vector<mll>;
using vvmll = std::vector<vmll>;
using vvvmll = std::vector<vvmll>;
using vvvvmll = std::vector<vvvmll>;

// a^n mod を計算する
LL modpow(LL a, LL n, LL mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

template <typename TTT> vector<LL> arg_sort(vector<TTT> A, bool ascend = true) {
  vector<LL> index(A.size());
  iota(index.begin(), index.end(), 0);
  if (ascend) {
    std::sort(index.begin(), index.end(),
              [&A](TTT i1, TTT i2) { return A[i1] < A[i2]; });
  } else {
    std::sort(index.begin(), index.end(),
              [&A](TTT i1, TTT i2) { return A[i1] > A[i2]; });
  }

  return index;
}

template <typename _Iterator, typename _Compare>
LL num_of_ika(_Iterator _first, _Iterator _last, _Compare key) {
  return (LL)(upper_bound(_first, _last, key) - _first);
}
template <typename _Iterator, typename _Compare>
LL num_of_ookii(_Iterator _first, _Iterator _last, _Compare key) {
  return (LL)(_last - upper_bound(_first, _last, key));
}
template <typename _Iterator, typename _Compare>
LL num_of_chisai(_Iterator _first, _Iterator _last, _Compare key) {
  return (LL)(lower_bound(_first, _last, key) - _first);
}
template <typename _Iterator, typename _Compare>
LL num_of_ijou(_Iterator _first, _Iterator _last, _Compare key) {
  return (LL)(_last - lower_bound(_first, _last, key));
}
template <typename _Iterator, typename _Compare>
LL num_of_onaji(_Iterator _first, _Iterator _last, _Compare key) {
  return (LL)(upper_bound(_first, _last, key) -
              lower_bound(_first, _last, key));
}

struct UnionFindTree {
private:
  vector<LL> UFT;

public:
  UnionFindTree(LL N) {
    UFT.resize(N);
    for (LL i = 0; i < N; i++) {
      UFT[i] = -1;
    }
  }

  LL root(LL i) {
    if (UFT[i] < 0) {
      return i;
    } else {
      LL j = root(UFT[i]);
      UFT[i] = j;
      return j;
    }
  }

  bool same(LL i, LL j) { return (root(i) == root(j)); }

  bool unite(LL i, LL j) {
    if (same(i, j)) {
      return false;
    }

    LL root_i = root(i);
    LL root_j = root(j);
    if (root_i != root_j) {
      if (size(root_i) < size(root_j)) {
        swap(root_i, root_j);
      }
      UFT[root_i] += UFT[root_j];
      UFT[root_j] = root_i;
    }
    return true;
  }

  LL size(LL i) { return -UFT[root(i)]; }

  LL get_root_num() {
    set<LL> roots;
    for (LL i = 0; i < (LL)UFT.size(); ++i) {
      roots.insert(root(i));
    }
    return (LL)roots.size();
  }

  map<LL, vector<LL>> get_root_child() {
    map<LL, vector<LL>> a;
    for (LL i = 0; i < (LL)UFT.size(); ++i) {
      LL j = root(i);
      a[j].push_back(i);
    }
    return a;
  }

  void print() {
    for (LL i = 0; i < (LL)UFT.size(); ++i) {
      cerr << root(i) << " ";
    }
    cerr << endl;
  }
};

#define cmax(a, b)                                                             \
  {                                                                            \
    if (a < b) {                                                               \
      a = b;                                                                   \
    }                                                                          \
  }
#define cmin(a, b)                                                             \
  {                                                                            \
    if (a > b) {                                                               \
      a = b;                                                                   \
    }                                                                          \
  }
#define EACH(a, A) for (auto a : A)
// #define cmax(a,b) a = (a>b ? a:b)
// #define cmin(a,b) a = (a<b ? a:b)
// cin,cout高速化のおまじない＋桁数指定
inline VLL cinvll(LL N, LL minus = 0) {
  VLL A(N);
  REP(i, N) {
    cin >> A[i];
    A[i] -= minus;
  }
  return move(A);
}
inline VVLL zerosll(LL H, LL W, LL val = 0) {
  VVLL A(H, VLL(W, val));
  return move(A);
}

#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)

// 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;
}
// vector
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {

  for (LL i = 0; i < (LL)vec.size(); i++) {
    // os << setw(3) <<i <<" : "<< setw(5) << vec[i] << endl;
    os << setw(5) << vec[i];
  }
  os << endl;
  return os;

#if 0    
    os << "{";
    for (LL i = 0; i < (LL)vec.size(); i++) {
        os << vec[i] << (i + 1 == (LL)vec.size() ? "" : ", ");
    }
    os << "}";
    return os;
#endif
}
// map
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var) {
  os << "{";
  repi(itr, map_var) {
    os << *itr;
    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;
    itr++;
    if (itr != set_var.end())
      os << ", ";
    itr--;
  }
  os << "}";

  return os;
}

#define DUMPOUT cerr
#define DUMPCOLOR ("\033[36m")

void dump_func() { DUMPOUT << endl; }
template <class Head, class... Tail>
void dump_func(Head &&head, Tail &&...tail) {
  DUMPOUT << DUMPCOLOR;
  DUMPOUT << head;
  if (sizeof...(Tail) > 0) {
    DUMPOUT << ", ";
  }
  dump_func(std::move(tail)...);
  DUMPOUT << "\033[m";
}
#ifdef DEBUG_
#define DEB
#define dump(...)                                                              \
  DUMPOUT << DUMPCOLOR << "" << string(#__VA_ARGS__) << ": "                   \
          << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]" << endl  \
          << "\033[m"                                                          \
          << "",                                                               \
      dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif

struct Fast {
  Fast() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(std::numeric_limits<double>::max_digits10);
  }
} fast;

#if 0 
LL GCD(LL m, LL n) {

  if (m < n) {
    swap(m, n);
  }

  while (n != 0) {
    LL n_new = m % n;
    m = n;
    n = n_new;
  }

  return m;

}

VLL B;
VVLL T;
LL cnt=0;
void dfs(LL me,LL oya){

  for(LL i=0;i<T[me].size();i++){
    LL ko = T[me][i];
    if(ko==oya){
      continue;
    }
    cnt += min(B[me],B[ko]);
    dfs(ko,me);
  }

}
#endif

bool isPrime(LL N) {
  if (N < 2) {
    return false;
  } else if (N == 2) {
    return true;
  } else if (N % 2 == 0) {
    return false;
  }

  LD sqrtNum = sqrt(N);
  for (LL i = 3; i <= sqrtNum; i += 2) {
    if (N % i == 0) {
      return false;
    }
  }
  return true;
}

#if 0
VLL A;
LL N,M;
// index が条件を満たすかどうか
bool isOK(LL X) {

  LL cnt=0;
  REP(i,N){
    LL hidari=A[i];
    auto it = lower_bound(ALL(A),X-hidari);
    cnt += A.end()-it;
  }
  dump(cnt);
  if(cnt>=M){
    return true;
  }
  else{
    return false;
  }
}
// 汎用的な二分探索のテンプレ
LL binary_search() {
  LL ng = 1e10+1 ;
  LL ok = 0;

  /* ok と ng のどちらが大きいかわからないことを考慮 */
  while (abs(ok - ng) > 1) {
      LL mid = (ok + ng) / 2;

      if (isOK(mid)) ok = mid;
      else ng = mid;
  }
  return ok;
}

map< LL, LL > prime_factor(LL n) {
  map< LL,LL > ret;
  for(LL i = 2; i * i <= n; i++) {
    while(n % i == 0) {
      ret[i]++;
      n /= i;
    }
  }
  if(n != 1) ret[n] = 1;
  return ret;
}

LL GCD(LL m, LL n) { 
  if (m < n) {
    swap(m, n);
  }

  while (n != 0) {
    LL n_new = m % n;
    m = n;
    n = n_new;
  }
  return m;
}
LL LCM(LL x, LL y) { 
  return x / GCD(x, y) * y; 
}

#endif

struct Furui {
  LL n;
  VLL f;
  VLL prime;

  Furui(LL n = 1) : n(n), f(n + 1) {
    f[0] = -1;
    f[1] = -1;
    for (LL i = 2; i <= n; i++) {
      if (f[i]) {
        continue;
      }
      prime.push_back(i);
      for (LL j = i; j <= n; j += i) {
        if (f[j] == 0) {
          f[j] = i;
        }
      }
    }
  }
  bool isPrime(LL x) {
    if (x <= 1) {
      return false;
    }
    return (f[x] == x);
  }
  VLL factorList(LL x) {
    VLL res;
    if (x >= 2 and x <= n) {
      while (x != 1) {
        res.push_back(f[x]);
        x /= f[x];
      }
    }
    return res;
  }
  vector<pair<LL, LL>> factor(LL x) {
    VLL factors = factorList(x);
    vector<pair<LL, LL>> P;
    if (factors.size() == 0) {
      return P;
    }
    P.emplace_back(factors[0], 0);
    for (auto p : factors) {
      if (P.back().first == p) {
        P.back().second++;
      } else {
        P.emplace_back(p, 1);
      }
    }
    return P;
  }
};

template <typename T> struct edge {
  int src, to;
  T cost;
  edge(int to, T cost) : src(-1), to(to), cost(cost) {}
  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}
  edge &operator=(const int &x) {
    to = x;
    return *this;
  }

  operator int() const { return to; }
};
template <typename T> using Edges = vector<edge<T>>;
template <typename T> using WeightedGraph = vector<Edges<T>>;
using UnWeightedGraph = vector<vector<int>>;
template <typename T> using Matrix = vector<vector<T>>;

template <typename T> void warshall_floyd(Matrix<T> &g, T INF) {
  for (int k = 0; k < g.size(); k++) {
    for (int i = 0; i < g.size(); i++) {
      for (int j = 0; j < g.size(); j++) {
        if (g[i][k] == INF || g[k][j] == INF)
          continue;
        g[i][j] = min(g[i][j], g[i][k] + g[k][j]);
      }
    }
  }
}

template <typename T> T kruskal(Edges<T> &edges, LL V) {
  sort(begin(edges), end(edges),
       [](const edge<T> &a, const edge<T> &b) { return (a.cost < b.cost); });
  UnionFindTree tree(V);
  T ret = 0;
  for (auto &e : edges) {
    if (tree.unite(e.src, e.to))
      ret += e.cost;
  }
  return (ret);
}

map<LL, LL> prime_factor(LL n) {
  map<LL, LL> ret;
  for (LL i = 2; i * i <= n; i++) {
    while (n % i == 0) {
      ret[i]++;
      n /= i;
    }
  }
  if (n != 1)
    ret[n] = 1;
  return ret;
}

#if 0
VLL A;
VLL B;
  
bool isOK(LL X) {


  LL cnt=0;
  REP(i,N){
    LL hidari=A[i];
    auto it = lower_bound(ALL(A),X-hidari);
    cnt += A.end()-it;
  }
  dump(cnt);
  if(cnt>=M){
    return true;
  }
  else{
    return false;
  }
}
// 汎用的な二分探索のテンプレ
LL binary_search() {
  LL ng = 0 ;
  LL ok = 100000;

  /* ok と ng のどちらが大きいかわからないことを考慮 */
  while (abs(ok - ng) > 1) {
      LL mid = (ok + ng) / 2;

      if (isOK(mid)) ok = mid;
      else ng = mid;
  }
  return ok;
}

void funcE()
{
  LL H,N;
  cin >>H >> N;
  VLL A;
  VLL B;
  A.resize(N);
  B.resize(N);
  REP(i,N){
    cin >> A[i];
    cin >> B[i];
  }
  dump(H,N);
  dump(A);
  dump(B);
}
#endif
LL powll(LL a, LL b) {
  LL c = 1LL;
  REP(i, b) { c *= a; }
  return c;
}

LL knapsack(vector<LL> v, vector<LL> w, LL W) {
  LL N = (LL)v.size();
  vector<LL> dp(W + 1, 0);
  for (LL i = 0; i < N; ++i) {
    for (LL j = w[i]; j <= W; ++j) {
      dp[j] = max(dp[j], dp[j - w[i]] + v[i]);
    }
  }

  return dp[W];
}

#if 0 

LL H,W;
VVLL M;
VVLL V;
LL sx;
LL sy;
LL gx;
LL gy;

void MeiroDFS(LL y,LL x){

  if(V[y][x]==1){
    return;
  }
  V[y][x]=1;

  if(y>0 and M[y-1][x]==0){
    dfs(y-1,x);
  }
  if(x>0 and M[y][x-1]==0){
    dfs(y,x-1);
  }
  if(y+1<H and M[y+1][x]==0){
    dfs(y+1,x);
  }
  if(x+1<W and M[y][x+1]==0){
    dfs(y,x+1);
  }

}

LL H,W;
VVLL M;
VVLL V;
LL sx;
LL sy;
LL gx;
LL gy;
void MeiroBFS(LL y,LL x){

  queue<tuple<LL,LL,LL>> Q;
  Q.emplace(y,x,0);
  V[y][x]=0;

  LL cnt=0;
  while(not Q.empty()){
    tuple<LL,LL,LL> q = Q.front();
    Q.pop();
    y = get<0>(q);
    x = get<1>(q);
    cnt = get<2>(q);
    if(y>0 and M[y-1][x]==0 and V[y-1][x]==-1){
      V[y-1][x]=cnt+1;
      Q.emplace(y-1,x,cnt+1);
    }
    if(x>0 and M[y][x-1]==0 and V[y][x-1]==-1){
      V[y][x-1]=cnt+1;
      Q.emplace(y,x-1,cnt+1);
    }
    if(y+1<H and M[y+1][x]==0 and V[y+1][x]==-1){
      V[y+1][x]=cnt+1;
      Q.emplace(y+1,x,cnt+1);
    }
    if(x+1<W and M[y][x+1]==0 and V[y][x+1]==-1){
      V[y][x+1]=cnt+1;
      Q.emplace(y,x+1,cnt+1);
    }
  }



}
#endif

template <typename T> vector<T> dijkstra(WeightedGraph<T> &g, int s) {
  const auto INF = numeric_limits<T>::max();
  vector<T> dist(g.size(), INF);

  using Pi = pair<T, int>;
  priority_queue<Pi, vector<Pi>, greater<Pi>> que;
  dist[s] = 0;
  que.emplace(dist[s], s);
  while (!que.empty()) {
    T cost;
    int idx;
    tie(cost, idx) = que.top();
    que.pop();
    if (dist[idx] < cost)
      continue;
    for (auto &e : g[idx]) {
      auto next_cost = cost + e.cost;
      if (dist[e.to] <= next_cost)
        continue;
      dist[e.to] = next_cost;
      que.emplace(dist[e.to], e.to);
    }
  }
  return dist;
}

template <typename T> struct BinaryIndexedTree {
  vector<T> data;

  BinaryIndexedTree(int sz = 10) { data.assign(++sz, 0); }

  T sum(int k) {
    T ret = 0;
    for (++k; k > 0; k -= k & -k)
      ret += data[k];
    return (ret);
  }

  void add(int k, T x) {
    for (++k; k < data.size(); k += k & -k)
      data[k] += x;
  }
  void resize(int sz) { data.assign(++sz, 0); }
};

bool isKaibun2(string S) {
  LL N = (LL)S.length();
  REP(i, (LL)(N / 2)) {
    if (S[i] != S[N - 1 - i]) {
      return false;
    }
  }
  return true;
}

LL Combination(LL a, LL b) {
  chmin(b, a - b);
  if (b < 0)
    return 0;
  LL c = 1;
  REP(i, b) c *= a - i;
  REP(i, b) c /= i + 1;
  return c;
}

struct IntegralImage {
  VVLL I;
  LL H;
  LL W;
  IntegralImage(VVLL M) {
    I = M;
    H = M.size();
    W = M[0].size();
    // check
    bool isOK = true;
    REP(i, H) {
      if (I[i].size() != W) {
        isOK = false;
      }
    }
    if (not isOK) {
      cerr << "shape error :";
      REP(i, H) { cerr << I[i].size() << " "; }
      cerr << endl;
      throw 0;
    }

    REP(i, H) {
      REP(j, W - 1) { I[i][j + 1] += I[i][j]; }
    }
    REP(j, W) {
      REP(i, H - 1) { I[i + 1][j] += I[i][j]; }
    }
  }
  LL get_sum(LL y, LL h, LL x, LL w) {
    if (y < 0 or x < 0 or h < 0 or w < 0 or y + h >= H or x + w >= W) {
      fprintf(stderr, "H=%lld, W=%lld, y=%lld, h=%lld, x=%lld, w=%lld\n", H, W,
              y, h, x, w);
      throw 1;
    }
    LL a1 = I[y + h][x + w];
    LL a2 = 0;
    if (y > 0) {
      a2 = I[y - 1][x + w];
    }
    LL a3 = 0;
    if (x > 0) {
      a3 = I[y + h][x - 1];
    }
    LL a4 = 0;
    if (y > 0 && x > 0) {
      a4 = I[y - 1][x - 1];
    }
    return (a1 + a4 - a2 - a3);
  }
  void print() {
    REP(i, (LL)I.size()) { PRINTSP(I[i]); }
  }
};

bool isKaibun(string S) {
  bool ok = true;
  LL N = (LL)S.length();
  REP(i, N) {
    if (S[i] != S[N - i - 1]) {
      ok = false;
      break;
    }
  }
  return ok;
}

void func() {
  string S;
  LL K;
  cin >> S;
  cin >> K;

  dump(S, K);

  set<string> T;
  LL N = S.length();
  REP(i, N) {
    REPM(j, min(N, i + 5), i) {
      string s = S.substr(i, j - i + 1);
      // Xdump(s);
      T.insert(s);
    }
  }

  auto t = T.begin();
  REP(k, K - 1) { t++; }
  bye(*t);

  return;
}
int main() {
  // funcD();
  func();
  return 0;
}