#include <bits/stdc++.h>
const int MOD = (int)1e9+7;using namespace std;using ll = long long;using ld = long double;using P = pair<ll,ll>;const string rt = "\n", sp = " ";/* GCD */template<typename M, typename N> constexpr common_type_t<M,N> gcd(M a, N b){return b != 0 ? gcd(b, a % b) : a;}/* LCM */template<typename M, typename N> constexpr common_type_t<M,N> lcm(M a, N b){return a*b/gcd(a,b);}/* UnionFind */template<typename T> struct UnionFind{vector<T> par;UnionFind(T n) : par(n, -1){}void init(T n){par.assign(n, -1);}T root(T x){if (par[x] < 0) return x;else return par[x] = root(par[x]);}bool issame(T x, T y){return root(x) == root(y);}bool merge(T x, T y){x = root(x); y = root(y);if(x == y) return false;if(par[x] > par[y]) swap(x, y);par[x] += par[y];par[y] = x;return true;}int size(int x) {return -par[root(x)];}};/* COMB */ll comb(ll n, ll r) {if(n < r) return 0;if ( r * 2 > n ) r = n - r;ll dividend = 1;ll divisor  = 1;for (ll i = 1; i <= r; ++i ) {dividend *= (n-i+1);divisor  *= i;}return dividend / divisor;}/* COMB % MOD */template<typename T> ll combpm(T N_, T C_) {const int NUM_=400001;static ll fact[NUM_+1],factr[NUM_+1],inv[NUM_+1];if (fact[0]==0) {inv[1]=fact[0]=factr[0]=1;for (int i=2;i<=NUM_;++i) inv[i] = inv[MOD % i] * (MOD - MOD / i) % MOD;for (int i=1;i<=NUM_;++i) fact[i]=fact[i-1]*i%MOD, factr[i]=factr[i-1]*inv[i]%MOD;}if(C_<0 || C_>N_) return 0;return factr[C_]*fact[N_]%MOD*factr[N_-C_]%MOD;}/* MAKE VECTOR */template<class T> constexpr vector<T> mvec(size_t a){return vector<T>(a);}template<class T, class... Ts>auto mvec(size_t a, Ts... ts){return vector<decltype(mvec<T>(ts...))>(a, mvec<T>(ts...));}/* MAKE DEQUE */template<class T>constexpr deque<T> mdeq(size_t a){return deque<T>(a);}template<class T, class... Ts>auto mdeq(size_t a, Ts... ts){return deque<decltype(mdeq<T>(ts...))>(a, mdeq<T>(ts...));}/* TEST */void test(ll n){cout << "test" << n << endl;}/* PRECISION */void fixsp(ll n){cout << fixed << setprecision(n);}void defsp(ll n){cout << defaultfloat << setprecision(n);}/* WEIGHTENED UnionFind */struct WUnionFind {vector<int> par;vector<int> rank;WUnionFind(int n = 1) {init(n);}void init(int n = 1) {par.resize(n); rank.resize(n);for (int i = 0; i < n; ++i) par[i] = i, rank[i] = 0;}int root(int x) {if (par[x] == x) {return x;}else {int r = root(par[x]);return par[x] = r;}}bool issame(int x, int y) {return root(x) == root(y);}bool merge(int x, int y) {x = root(x); y = root(y);if (x == y) return false;if (rank[x] < rank[y]) swap(x, y);if (rank[x] == rank[y]) ++rank[x];par[y] = x;return true;}};/* DIVISOR */deque<ll> divisor(ll n) {deque<ll> ret;for(ll i = 1; i * i <= n; i++) {if(n % i == 0) {ret.push_back(i);if(i * i != n) ret.push_back(n / i);}}sort(begin(ret), end(ret));return (ret);}/* MODE */template<typename T> auto mode(T data){if(data.size() == 1) return *begin(data);sort(begin(data), end(data));typename decltype(data)::value_type mode{};size_t n{},count{1};for(auto iter = adjacent_find(begin(data), end(data)),last = end(data), next = end(data); iter != last;){next = upper_bound(iter, last, *iter);count = distance(iter,next);if(n < count) n = count, mode = *iter;iter = adjacent_find(next, last);}return mode;}/* MEDIAN */template<typename T> auto median(T data){sort(begin(data), end(data));size_t median_index = data.size() / 2;return (data.size() % 2 == 0? static_cast<double>(data[median_index] + data[median_index - 1]) / 2: data[median_index]);}/* INT POW */template<typename T> ll multi(T a, T b){ll ans{1};for(int i{}; i<b; i++) ans *= a;return ans;}/* INF */template<typename T> constexpr T inf(){return numeric_limits<T>::max();}/* FASTER IO */void fastio(){ios::sync_with_stdio(false);cin.tie(NULL);}/* MIN COST FLOW */template< typename flow_t, typename cost_t >struct PrimalDual {const cost_t INF;struct edge {int to;flow_t cap;cost_t cost;int rev;bool isrev;};vector< vector< edge > > graph;vector< cost_t > potential, min_cost;vector< int > prevv, preve;PrimalDual(int V) : graph(V), INF(numeric_limits< cost_t >::max()) {}void add_edge(int from, int to, flow_t cap, cost_t cost) {graph[from].emplace_back((edge) {to, cap, cost, (int) graph[to].size(), false});graph[to].emplace_back((edge) {from, 0, -cost, (int) graph[from].size() - 1, true});}cost_t min_cost_flow(int s, int t, flow_t f) {int V = (int) graph.size();cost_t ret = 0;using Pi = pair< cost_t, int >;priority_queue< Pi, vector< Pi >, greater< Pi > > que;potential.assign(V, 0);preve.assign(V, -1);prevv.assign(V, -1);while(f > 0) {min_cost.assign(V, INF);que.emplace(0, s);min_cost[s] = 0;while(!que.empty()) {Pi p = que.top();que.pop();if(min_cost[p.second] < p.first) continue;for(int i = 0; i < graph[p.second].size(); i++) {edge &e = graph[p.second][i];cost_t nextCost = min_cost[p.second] + e.cost + potential[p.second] - potential[e.to];if(e.cap > 0 && min_cost[e.to] > nextCost) {min_cost[e.to] = nextCost;prevv[e.to] = p.second, preve[e.to] = i;que.emplace(min_cost[e.to], e.to);}}}if(min_cost[t] == INF) return -1;for(int v = 0; v < V; v++) potential[v] += min_cost[v];flow_t addflow = f;for(int v = t; v != s; v = prevv[v]) {addflow = min(addflow, graph[prevv[v]][preve[v]].cap);}f -= addflow;ret += addflow * potential[t];for(int v = t; v != s; v = prevv[v]) {edge &e = graph[prevv[v]][preve[v]];e.cap -= addflow;graph[v][e.rev].cap += addflow;}}return ret;}void output() {for(int i = 0; i < graph.size(); i++) {for(auto &e : graph[i]) {if(e.isrev) continue;auto &rev_e = graph[e.to][e.rev];cout << i << "->" << e.to << " (flow: " << rev_e.cap << "/" << rev_e.cap + e.cap << ")" << endl;}}}};
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define rep(i, s, N) for(ll i{s}; i < (N); i++)
#define rem(i, N, s) for(ll i{N}; i > (s); i--)

/* CODE STARTS FROM HERE */

int main(){
  fastio();
  string s;
  ll K;
  cin >> s >> K;
  auto dict = mdeq<string>(0);
  rep(i,0,s.size()) rep(j,1,K+1){
    dict.push_back(s.substr(i,j));
  }
  sort(all(dict));
  dict.erase(unique(all(dict)), dict.end());
  cout << dict[K-1] << rt;
}

Measured memory_bytes: 1664