Problem p03355 (C++)

Buggy submission (status: Time Limit Exceeded):
// #pragma once
#include <algorithm>
#include <bitset>
#include <cctype>
#include <cstdlib> // srand,rand
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <math.h>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stdio.h>
#include <stdlib.h>
#include <string>
#include <vector>

using namespace std;
#define ll long long
#define lp(i, n) for (ll i = 0; i < n; i++)

#define modd 1000000007
#define mod2 998244353
const double EPS = 1e-20;
const double PI = acos(-1);

#define INF 8223372036854775807ll
#define ALL(a) (a).begin(), (a).end()

typedef pair<long long, long long> pl;
typedef pair<double, double> pd;
typedef pair<ll, string> pls;

typedef string::const_iterator State;
class ParseError {};

/*
class SegmentTree {



private:

        ll cont_num = 2;
        pd initial_v;
        vector<pd> dat;


public:


        SegmentTree() {};


        void init(ll size, double initial_value_first, double
initial_value_second) {

                cont_num = 2;
                initial_v.first = initial_value_first;
                initial_v.second = initial_value_second;

                while (cont_num < size) {
                        cont_num *= 2;
                }

                dat.resize(2 * cont_num);//サイズ設定

                for (int i = 0; i < 2 * cont_num; i++)dat[i] =
initial_v;//初期化



        }


        void Update(ll position, double value_f, double value_s) {



                ll k = cont_num + position;

                dat[k].first = value_f;
                dat[k].second = value_s;

                while (k > 1) {
                        k /= 2;
                        dat[k].first = dat[k * 2 + 1].first * dat[k * 2].first;
                        dat[k].second = dat[k * 2 + 1].first * dat[k * 2].second
+ dat[k * 2 + 1].second;


                }


        }

        /*
        ll query_proces(ll a, ll b, ll k, ll l, ll r) {

                if (r <= a || b <= l)return initial_v;

                if (a <= l && r <= b)return dat[k];
                else {
                        ll vl = query_proces(a, b, k * 2, l, (l + r) / 2);
                        ll vr = query_proces(a, b, k * 2 + 1, (l + r) / 2, r);
                        return min(vl, vr);
                }

        }


        ll query(ll left, ll right) {
                return query_proces(left, right, 1, 0, cont_num);
        }
        */
/*
        double query() {
                return dat[1].first + dat[1].second;
        }

};*/

template <typename T> class Zip {
  vector<T> d;
  bool flag;
  void init() {
    sort(d.begin(), d.end());
    d.erase(unique(d.begin(), d.end()), d.end());
    flag = false;
  }

public:
  Zip() { flag = false; }
  void add(T x) {
    d.push_back(x);
    flag = true;
  }
  ll getNum(T x) {
    if (flag)
      init();
    return lower_bound(d.begin(), d.end(), x) - d.begin();
  }
  ll size() {
    if (flag)
      init();
    return (ll)d.size();
  }
};

ll N, M, K, a, b, c, d, e, H, W, L, T;
ll x, y, z;

bool f;
string SS;
set<long long> sll;
pl bufpl;
vector<long long> vll;
vector<long long> v;
vector<pl> vpl;
vector<string> vs;
set<ll> llset;
set<string> Sset;
multiset<ll> llmset;
queue<ll> ql;
multiset<pl> plmset;

typedef struct ST {
  ll first;
  ll second;
  ll cost;

  bool operator<(const ST &another) const {
    return cost < another.cost; // 比較
  };

  bool operator>(const ST &another) const {
    return cost > another.cost; // 比較
  };

} ST;
// queue<ST> qst;
// priority_queue<ST, vector<ST>, greater<ST> > qst;

/*vector <ST> vst;
ST st[200005];
ST bufst;
bitset<5000> bits;*/

long long modinv(long long aa, long long mm) {
  long long bb = mm, uu = 1, vv = 0;
  while (bb) {
    long long tt = aa / bb;
    aa -= tt * bb;
    swap(aa, bb);
    uu -= tt * vv;
    swap(uu, vv);
  }
  uu %= mm;
  if (uu < 0)
    uu += mm;
  return uu;
}

ll zettai(ll aa) {

  if (aa < 0) {
    aa *= -1;
  }
  return aa;
}

float zettai(float aa) {

  if (aa < 0) {
    aa *= -1;
  }
  return aa;
}

double zettai(double aa) {

  if (aa < 0) {
    aa *= -1;
  }
  return aa;
}

/*
class UnionFind
{

public:


        vector <ll>pairent;
        vector <ll>depth;
        vector <ll>size;


        UnionFind(ll Amount) : pairent(Amount, 1), depth(Amount, 1),
size(Amount, 1) {

                for (ll i = 0; i < Amount; i++) {
                        pairent[i] = i;
                }


        }


        ll FindPairent(ll x) {
                if (pairent[x] == x)return x;
                else return pairent[x] = FindPairent(pairent[x]);

        }


        ll Merge(ll x, ll y) {
                x = FindPairent(x);
                y = FindPairent(y);

                if (x != y) {
                        if (depth[x] > depth[y]) {
                                pairent[y] = pairent[x];
                                return size[x] += size[y];
                        }
                        else {
                                pairent[x] = pairent[y];
                                if (depth[x] == depth[y]) {
                                        depth[y] ++;
                                }

                                return size[y] += size[x];
                        }

                }
                else {
                        return -1;
                }



        }



        bool IsSame(ll x, ll y) {
                if (FindPairent(x) == FindPairent(y))return true;
                else return false;
        }


        ll GetSize(ll x) {
                x = FindPairent(x);
                return size[x];
        }


};
*/

class UnionFind {

public:
  vector<ll> pairent;
  vector<ll> depth;
  vector<ll> size;

  UnionFind(ll Amount) : pairent(Amount, 1), depth(Amount, 1), size(Amount, 1) {

    for (ll i = 0; i < Amount; i++) {
      pairent[i] = i;
    }
  }

  ll FindPairent(ll x) {
    if (pairent[x] == x)
      return x;
    else
      return pairent[x] = FindPairent(pairent[x]);
  }

  ll Merge(ll x, ll y) {
    x = FindPairent(x);
    y = FindPairent(y);

    if (x != y) {
      if (depth[x] > depth[y]) {
        pairent[y] = pairent[x];
        return size[x] += size[y];
      } else {
        pairent[x] = pairent[y];
        if (depth[x] == depth[y]) {
          depth[y]++;
        }
        return size[y] += size[x];
      }
    } else {
      return -1;
    }
  }

  bool IsSame(ll x, ll y) {
    if (FindPairent(x) == FindPairent(y))
      return true;
    else
      return false;
  }

  ll GetSize(ll x) {
    x = FindPairent(x);
    return size[x];
  }
};

struct Edge {
  ll a, b, cost;

  bool operator<(const Edge &other) const { return cost < other.cost; }
};

struct Graph {
  ll n;            // 頂点数
  vector<Edge> es; // 辺集合
};

class Kruskal {

  Graph origin_G;
  Graph MST;
  ll total_cost = 0;

public:
  void Solve() {
    UnionFind uf = UnionFind(MST.n);
    for (ll i = 0; i < origin_G.es.size(); i++) {
      ll a = origin_G.es[i].a;
      ll b = origin_G.es[i].b;
      ll cost = origin_G.es[i].cost;

      if (!uf.IsSame(a, b)) {
        uf.Merge(a, b);
        MST.es.push_back(origin_G.es[i]);
        total_cost += cost;
      }
    }
  }

  Kruskal(Graph graph) {
    origin_G = graph;
    MST = graph;
    MST.es.clear();
    sort(origin_G.es.begin(), origin_G.es.end());
  }

  ll GetMinCost() { return total_cost; }
};

ll RepeatSquaring(ll N, ll P, ll M) {
  if (P == 0)
    return 1;
  if (P % 2 == 0) {
    ll t = RepeatSquaring(N, P / 2, M) % M;
    return t * t % M;
  }
  return N * RepeatSquaring(N, P - 1, M) % M;
}

/*
ll KurikaesiNijou(ll a, ll b, ll P) {

        if (b == 0)return 1;
        if (b % 2 == 0) {
                ll c=KurikaesiNijou(a,b/2,P)%P;
                return c * c %P;
        }
        else {
                ll c = KurikaesiNijou(a, b/2, P) % P;
                return a * c * c % P;
        }

}*/

ll GCD(ll a, ll b) {
  if (a % b == 0)
    return b;
  else
    return GCD(b, a % b);
}

ll Min(ll a, ll b) {
  if (a < b)
    return a;
  else
    return b;
}

ll Max(ll a, ll b) {
  if (a > b)
    return a;
  else
    return b;
}

ll Sum(ll a, ll b) { return a + b; }

template <typename T> class SegmentTree {
  ll n;
  vector<T> node;
  function<T(T, T)> fun, fun2;
  bool customChange;
  T outValue, initValue;

public:
  void init(ll num, function<T(T, T)> resultFunction, T init, T out,
            function<T(T, T)> changeFunction = NULL) {
    // changeFunction: (input, beforevalue) => newvalue
    fun = resultFunction;
    fun2 = changeFunction;
    customChange = changeFunction != NULL;
    n = 1;
    while (n < num)
      n *= 2;
    node.resize(2 * n - 1);
    fill(node.begin(), node.end(), init);
    outValue = out;
    initValue = init;
  }
  void valueChange(ll num, T value) {
    num += n - 1;
    if (customChange)
      node[num] = fun2(value, node[num]);
    else
      node[num] = value;
    while (num > 0)
      num = (num - 1) / 2,
      node[num] = fun(node[num * 2 + 1], node[num * 2 + 2]);
  }
  T rangeQuery(ll a, ll b, ll l = 0, ll r = -1, ll k = 0) { // [a, b)
    if (r == -1)
      r = n;
    if (a <= l && r <= b)
      return node[k];
    if (b <= l || r <= a)
      return outValue;
    ll mid = (l + r) / 2;
    return fun(rangeQuery(a, b, l, mid, 2 * k + 1),
               rangeQuery(a, b, mid, r, 2 * k + 2));
  }
};

class Combination {

  vector<ll> factorial;
  vector<ll> factorial_inv;
  ll mod;
  ll max_n;

public:
  void Init(ll init_max_n, ll init_mod) {
    max_n = init_max_n;
    mod = init_mod;
    factorial.resize(max_n + 1);
    factorial[0] = 1;
    for (ll i = 1; i < factorial.size(); i++) {
      factorial[i] = factorial[i - 1] * i;
      factorial[i] %= mod;
    }

    factorial_inv.resize(max_n + 1);
    factorial_inv[0] = 1;
    for (ll i = 1; i < factorial_inv.size(); i++) {
      factorial_inv[i] = factorial_inv[i - 1] * modinv(i, mod);
      factorial_inv[i] %= mod;
    }
  }

  ll GetComb(ll n, ll k) {
    ll comb = factorial[n];
    comb *= factorial_inv[k];
    comb %= mod;
    comb *= factorial_inv[n - k];
    comb %= mod;
    return comb;
  }

  ll GetH(ll n, ll k) { // n+k-1<=max_N
    ll comb = factorial[n + k - 1];
    comb *= factorial_inv[n];
    comb %= mod;
    comb *= factorial_inv[k - 1];
    comb %= mod;
    return comb;
  }
};

class Tree {

  ll node_N;
  vector<ll> node;
  vector<vector<pl>> pass;
  ll diameter = -1;
  vector<ll> dist_Diamieter[2];

  pl maxDist_Num;

public:
  void Init(ll node_Num) {
    node_N = node_Num;
    node.resize(node_N + 1);
    pass.resize(node_N + 1);

    dist_Diamieter[0].resize(node_N + 1);
    lp(i, node_N + 1) dist_Diamieter[0][i] = -1;
    dist_Diamieter[1].resize(node_N + 1);
    lp(i, node_N + 1) dist_Diamieter[1][i] = -1;
  }

  void AddEdge(ll a, ll b, ll dist) {
    bufpl.first = b;
    bufpl.second = dist;
    pass[a].push_back(bufpl);
    bufpl.first = a;
    pass[b].push_back(bufpl);
  }

  void DFS(ll step, ll now, ll dist) {

    dist_Diamieter[step][now] = dist;
    if (dist_Diamieter[step][now] > maxDist_Num.first) {
      maxDist_Num.first = dist_Diamieter[step][now];
      maxDist_Num.second = now;
    }

    for (ll i = 0; i < pass[now].size(); i++) {
      ll next_node = pass[now][i].first;
      if (dist_Diamieter[step][next_node] == -1) {
        DFS(step, next_node, dist + pass[now][i].second);
      }
    }
  }

  ll GetDiameter(ll min_node_num) {
    if (diameter >= 0)
      return diameter;
    else {
      maxDist_Num.first = -1;
      maxDist_Num.second = -1;
      DFS(0, min_node_num, 0ll);
      ll step2_start = maxDist_Num.second;

      maxDist_Num.first = -1;
      maxDist_Num.second = -1;
      DFS(1, step2_start, 0ll);

      diameter = maxDist_Num.first;
      return diameter;
    }
  }

  ll GetDistFromMinNode(ll num) { return dist_Diamieter[0][num]; }
};

ll A[1000006] = {};
ll B[1000006] = {};
ll C[500005] = {};
Combination co;
map<ll, ll> llmap;
set<string> sset;

int main() {
  cin >> SS >> K;
  N = SS.size();
  a = 0;
  lp(i, 26) {
    lp(j, N) {
      if (SS[j] == 'a' + i) {
        string S;
        for (ll k = j; k < min(k + 5, N); k++) {
          S += SS[k];
          sset.insert(S);
        }
        for (auto itr = sset.end(); sset.size() > K; itr--) {
          if (itr == sset.end())
            itr--;
          itr = sset.erase(itr);
        }
      }
    }
    if (sset.size() == K)
      break;
  }
  auto itr = sset.end();
  itr--;
  cout << *itr << endl;

  // cout << fixed << setprecision(12) << bb << endl;
  return 0;
}


Fixed submission:
// #pragma once
#include <algorithm>
#include <bitset>
#include <cctype>
#include <cstdlib> // srand,rand
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <math.h>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stdio.h>
#include <stdlib.h>
#include <string>
#include <vector>

using namespace std;
#define ll long long
#define lp(i, n) for (ll i = 0; i < n; i++)

#define modd 1000000007
#define mod2 998244353
const double EPS = 1e-20;
const double PI = acos(-1);

#define INF 8223372036854775807ll
#define ALL(a) (a).begin(), (a).end()

typedef pair<long long, long long> pl;
typedef pair<double, double> pd;
typedef pair<ll, string> pls;

typedef string::const_iterator State;
class ParseError {};

/*
class SegmentTree {



private:

        ll cont_num = 2;
        pd initial_v;
        vector<pd> dat;


public:


        SegmentTree() {};


        void init(ll size, double initial_value_first, double
initial_value_second) {

                cont_num = 2;
                initial_v.first = initial_value_first;
                initial_v.second = initial_value_second;

                while (cont_num < size) {
                        cont_num *= 2;
                }

                dat.resize(2 * cont_num);//サイズ設定

                for (int i = 0; i < 2 * cont_num; i++)dat[i] =
initial_v;//初期化



        }


        void Update(ll position, double value_f, double value_s) {



                ll k = cont_num + position;

                dat[k].first = value_f;
                dat[k].second = value_s;

                while (k > 1) {
                        k /= 2;
                        dat[k].first = dat[k * 2 + 1].first * dat[k * 2].first;
                        dat[k].second = dat[k * 2 + 1].first * dat[k * 2].second
+ dat[k * 2 + 1].second;


                }


        }

        /*
        ll query_proces(ll a, ll b, ll k, ll l, ll r) {

                if (r <= a || b <= l)return initial_v;

                if (a <= l && r <= b)return dat[k];
                else {
                        ll vl = query_proces(a, b, k * 2, l, (l + r) / 2);
                        ll vr = query_proces(a, b, k * 2 + 1, (l + r) / 2, r);
                        return min(vl, vr);
                }

        }


        ll query(ll left, ll right) {
                return query_proces(left, right, 1, 0, cont_num);
        }
        */
/*
        double query() {
                return dat[1].first + dat[1].second;
        }

};*/

template <typename T> class Zip {
  vector<T> d;
  bool flag;
  void init() {
    sort(d.begin(), d.end());
    d.erase(unique(d.begin(), d.end()), d.end());
    flag = false;
  }

public:
  Zip() { flag = false; }
  void add(T x) {
    d.push_back(x);
    flag = true;
  }
  ll getNum(T x) {
    if (flag)
      init();
    return lower_bound(d.begin(), d.end(), x) - d.begin();
  }
  ll size() {
    if (flag)
      init();
    return (ll)d.size();
  }
};

ll N, M, K, a, b, c, d, e, H, W, L, T;
ll x, y, z;

bool f;
string SS;
set<long long> sll;
pl bufpl;
vector<long long> vll;
vector<long long> v;
vector<pl> vpl;
vector<string> vs;
set<ll> llset;
set<string> Sset;
multiset<ll> llmset;
queue<ll> ql;
multiset<pl> plmset;

typedef struct ST {
  ll first;
  ll second;
  ll cost;

  bool operator<(const ST &another) const {
    return cost < another.cost; // 比較
  };

  bool operator>(const ST &another) const {
    return cost > another.cost; // 比較
  };

} ST;
// queue<ST> qst;
// priority_queue<ST, vector<ST>, greater<ST> > qst;

/*vector <ST> vst;
ST st[200005];
ST bufst;
bitset<5000> bits;*/

long long modinv(long long aa, long long mm) {
  long long bb = mm, uu = 1, vv = 0;
  while (bb) {
    long long tt = aa / bb;
    aa -= tt * bb;
    swap(aa, bb);
    uu -= tt * vv;
    swap(uu, vv);
  }
  uu %= mm;
  if (uu < 0)
    uu += mm;
  return uu;
}

ll zettai(ll aa) {

  if (aa < 0) {
    aa *= -1;
  }
  return aa;
}

float zettai(float aa) {

  if (aa < 0) {
    aa *= -1;
  }
  return aa;
}

double zettai(double aa) {

  if (aa < 0) {
    aa *= -1;
  }
  return aa;
}

/*
class UnionFind
{

public:


        vector <ll>pairent;
        vector <ll>depth;
        vector <ll>size;


        UnionFind(ll Amount) : pairent(Amount, 1), depth(Amount, 1),
size(Amount, 1) {

                for (ll i = 0; i < Amount; i++) {
                        pairent[i] = i;
                }


        }


        ll FindPairent(ll x) {
                if (pairent[x] == x)return x;
                else return pairent[x] = FindPairent(pairent[x]);

        }


        ll Merge(ll x, ll y) {
                x = FindPairent(x);
                y = FindPairent(y);

                if (x != y) {
                        if (depth[x] > depth[y]) {
                                pairent[y] = pairent[x];
                                return size[x] += size[y];
                        }
                        else {
                                pairent[x] = pairent[y];
                                if (depth[x] == depth[y]) {
                                        depth[y] ++;
                                }

                                return size[y] += size[x];
                        }

                }
                else {
                        return -1;
                }



        }



        bool IsSame(ll x, ll y) {
                if (FindPairent(x) == FindPairent(y))return true;
                else return false;
        }


        ll GetSize(ll x) {
                x = FindPairent(x);
                return size[x];
        }


};
*/

class UnionFind {

public:
  vector<ll> pairent;
  vector<ll> depth;
  vector<ll> size;

  UnionFind(ll Amount) : pairent(Amount, 1), depth(Amount, 1), size(Amount, 1) {

    for (ll i = 0; i < Amount; i++) {
      pairent[i] = i;
    }
  }

  ll FindPairent(ll x) {
    if (pairent[x] == x)
      return x;
    else
      return pairent[x] = FindPairent(pairent[x]);
  }

  ll Merge(ll x, ll y) {
    x = FindPairent(x);
    y = FindPairent(y);

    if (x != y) {
      if (depth[x] > depth[y]) {
        pairent[y] = pairent[x];
        return size[x] += size[y];
      } else {
        pairent[x] = pairent[y];
        if (depth[x] == depth[y]) {
          depth[y]++;
        }
        return size[y] += size[x];
      }
    } else {
      return -1;
    }
  }

  bool IsSame(ll x, ll y) {
    if (FindPairent(x) == FindPairent(y))
      return true;
    else
      return false;
  }

  ll GetSize(ll x) {
    x = FindPairent(x);
    return size[x];
  }
};

struct Edge {
  ll a, b, cost;

  bool operator<(const Edge &other) const { return cost < other.cost; }
};

struct Graph {
  ll n;            // 頂点数
  vector<Edge> es; // 辺集合
};

class Kruskal {

  Graph origin_G;
  Graph MST;
  ll total_cost = 0;

public:
  void Solve() {
    UnionFind uf = UnionFind(MST.n);
    for (ll i = 0; i < origin_G.es.size(); i++) {
      ll a = origin_G.es[i].a;
      ll b = origin_G.es[i].b;
      ll cost = origin_G.es[i].cost;

      if (!uf.IsSame(a, b)) {
        uf.Merge(a, b);
        MST.es.push_back(origin_G.es[i]);
        total_cost += cost;
      }
    }
  }

  Kruskal(Graph graph) {
    origin_G = graph;
    MST = graph;
    MST.es.clear();
    sort(origin_G.es.begin(), origin_G.es.end());
  }

  ll GetMinCost() { return total_cost; }
};

ll RepeatSquaring(ll N, ll P, ll M) {
  if (P == 0)
    return 1;
  if (P % 2 == 0) {
    ll t = RepeatSquaring(N, P / 2, M) % M;
    return t * t % M;
  }
  return N * RepeatSquaring(N, P - 1, M) % M;
}

/*
ll KurikaesiNijou(ll a, ll b, ll P) {

        if (b == 0)return 1;
        if (b % 2 == 0) {
                ll c=KurikaesiNijou(a,b/2,P)%P;
                return c * c %P;
        }
        else {
                ll c = KurikaesiNijou(a, b/2, P) % P;
                return a * c * c % P;
        }

}*/

ll GCD(ll a, ll b) {
  if (a % b == 0)
    return b;
  else
    return GCD(b, a % b);
}

ll Min(ll a, ll b) {
  if (a < b)
    return a;
  else
    return b;
}

ll Max(ll a, ll b) {
  if (a > b)
    return a;
  else
    return b;
}

ll Sum(ll a, ll b) { return a + b; }

template <typename T> class SegmentTree {
  ll n;
  vector<T> node;
  function<T(T, T)> fun, fun2;
  bool customChange;
  T outValue, initValue;

public:
  void init(ll num, function<T(T, T)> resultFunction, T init, T out,
            function<T(T, T)> changeFunction = NULL) {
    // changeFunction: (input, beforevalue) => newvalue
    fun = resultFunction;
    fun2 = changeFunction;
    customChange = changeFunction != NULL;
    n = 1;
    while (n < num)
      n *= 2;
    node.resize(2 * n - 1);
    fill(node.begin(), node.end(), init);
    outValue = out;
    initValue = init;
  }
  void valueChange(ll num, T value) {
    num += n - 1;
    if (customChange)
      node[num] = fun2(value, node[num]);
    else
      node[num] = value;
    while (num > 0)
      num = (num - 1) / 2,
      node[num] = fun(node[num * 2 + 1], node[num * 2 + 2]);
  }
  T rangeQuery(ll a, ll b, ll l = 0, ll r = -1, ll k = 0) { // [a, b)
    if (r == -1)
      r = n;
    if (a <= l && r <= b)
      return node[k];
    if (b <= l || r <= a)
      return outValue;
    ll mid = (l + r) / 2;
    return fun(rangeQuery(a, b, l, mid, 2 * k + 1),
               rangeQuery(a, b, mid, r, 2 * k + 2));
  }
};

class Combination {

  vector<ll> factorial;
  vector<ll> factorial_inv;
  ll mod;
  ll max_n;

public:
  void Init(ll init_max_n, ll init_mod) {
    max_n = init_max_n;
    mod = init_mod;
    factorial.resize(max_n + 1);
    factorial[0] = 1;
    for (ll i = 1; i < factorial.size(); i++) {
      factorial[i] = factorial[i - 1] * i;
      factorial[i] %= mod;
    }

    factorial_inv.resize(max_n + 1);
    factorial_inv[0] = 1;
    for (ll i = 1; i < factorial_inv.size(); i++) {
      factorial_inv[i] = factorial_inv[i - 1] * modinv(i, mod);
      factorial_inv[i] %= mod;
    }
  }

  ll GetComb(ll n, ll k) {
    ll comb = factorial[n];
    comb *= factorial_inv[k];
    comb %= mod;
    comb *= factorial_inv[n - k];
    comb %= mod;
    return comb;
  }

  ll GetH(ll n, ll k) { // n+k-1<=max_N
    ll comb = factorial[n + k - 1];
    comb *= factorial_inv[n];
    comb %= mod;
    comb *= factorial_inv[k - 1];
    comb %= mod;
    return comb;
  }
};

class Tree {

  ll node_N;
  vector<ll> node;
  vector<vector<pl>> pass;
  ll diameter = -1;
  vector<ll> dist_Diamieter[2];

  pl maxDist_Num;

public:
  void Init(ll node_Num) {
    node_N = node_Num;
    node.resize(node_N + 1);
    pass.resize(node_N + 1);

    dist_Diamieter[0].resize(node_N + 1);
    lp(i, node_N + 1) dist_Diamieter[0][i] = -1;
    dist_Diamieter[1].resize(node_N + 1);
    lp(i, node_N + 1) dist_Diamieter[1][i] = -1;
  }

  void AddEdge(ll a, ll b, ll dist) {
    bufpl.first = b;
    bufpl.second = dist;
    pass[a].push_back(bufpl);
    bufpl.first = a;
    pass[b].push_back(bufpl);
  }

  void DFS(ll step, ll now, ll dist) {

    dist_Diamieter[step][now] = dist;
    if (dist_Diamieter[step][now] > maxDist_Num.first) {
      maxDist_Num.first = dist_Diamieter[step][now];
      maxDist_Num.second = now;
    }

    for (ll i = 0; i < pass[now].size(); i++) {
      ll next_node = pass[now][i].first;
      if (dist_Diamieter[step][next_node] == -1) {
        DFS(step, next_node, dist + pass[now][i].second);
      }
    }
  }

  ll GetDiameter(ll min_node_num) {
    if (diameter >= 0)
      return diameter;
    else {
      maxDist_Num.first = -1;
      maxDist_Num.second = -1;
      DFS(0, min_node_num, 0ll);
      ll step2_start = maxDist_Num.second;

      maxDist_Num.first = -1;
      maxDist_Num.second = -1;
      DFS(1, step2_start, 0ll);

      diameter = maxDist_Num.first;
      return diameter;
    }
  }

  ll GetDistFromMinNode(ll num) { return dist_Diamieter[0][num]; }
};

ll A[1000006] = {};
ll B[1000006] = {};
ll C[500005] = {};
Combination co;
map<ll, ll> llmap;
set<string> sset;

int main() {
  cin >> SS >> K;
  N = SS.size();
  a = 0;
  lp(i, 26) {
    lp(j, N) {
      if (SS[j] == 'a' + i) {
        string S;
        for (ll k = j; k < min(j + 5, N); k++) {
          S += SS[k];
          sset.insert(S);
        }
        for (auto itr = sset.end(); sset.size() > K; itr--) {
          if (itr == sset.end())
            itr--;
          itr = sset.erase(itr);
        }
      }
    }
    if (sset.size() == K)
      break;
  }
  auto itr = sset.end();
  itr--;
  cout << *itr << endl;

  // cout << fixed << setprecision(12) << bb << endl;
  return 0;
}