#include "bits/stdc++.h"
#pragma GCC optimize("Ofast")

// Begin Header {{{
using namespace std;

#ifndef DEBUG
#define dump(...)
#endif

#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define rep(i, n) for (intmax_t i = 0, i##_limit = (n); i < i##_limit; ++i)
#define reps(i, b, e) for (intmax_t i = (b), i##_limit = (e); i <= i##_limit; ++i)
#define repr(i, b, e) for (intmax_t i = (b), i##_limit = (e); i >= i##_limit; --i)
#define var(Type, ...) Type __VA_ARGS__; input(__VA_ARGS__)

constexpr size_t operator""_zu(unsigned long long value) { return value; };
constexpr intmax_t operator""_jd(unsigned long long value) { return value; };
constexpr uintmax_t operator""_ju(unsigned long long value) { return value; };

constexpr int INF = 0x3f3f3f3f;
constexpr intmax_t LINF = 0x3f3f3f3f3f3f3f3f_jd;

namespace moke {

    template <class T> using MaxHeap = priority_queue<T, vector<T>, less<T>>;
    template <class T> using MinHeap = priority_queue<T, vector<T>, greater<T>>;

    inline void input() {}
    template <class Head, class... Tail> inline void input(Head&& head, Tail&&... tail) {
        cin >> head;
        input(forward<Tail>(tail)...);
    }

    inline void outs() { cout << "\n"; }
    template <class Head, class... Tail> inline void outs(Head&& head, Tail&&... tail) {
        cout << head << (sizeof...(tail) ? " " : "");
        outs(forward<Tail>(tail)...);
    }

    template <class T> inline void outs(vector<T> &vec) {
        for (auto &e : vec) cout << e << " \n"[&e == &vec.back()];
    }

    template <class T> inline void outs(vector<vector<T>> &df) {
        for (auto &vec : df) outs(vec);
    }

    inline void outl() { cout << "\n"; }
    template <class Head, class... Tail> inline void outl(Head&& head, Tail&&... tail) {
        cout << head << (sizeof...(tail) ? "\n" : "");
        outl(forward<Tail>(tail)...);
    }

    template <class T> inline void outl(vector<T> &vec) {
        for (auto &e : vec) cout << e << "\n";
    }

    inline void outn() {}
    template <class Head, class... Tail> inline void outn(Head&& head, Tail&&... tail) {
        cout << head;
        outn(forward<Tail>(tail)...);
    }

    template <class T> inline void outn(vector<T> &vec) {
        for (auto &e : vec) cout << e;
    }

    template <class T> inline vector<T> make_v(const T &initvalue, size_t sz) {
        return vector<T>(sz, initvalue);
    }

    template <class T, class... Args> inline auto make_v(const T &initvalue, size_t sz, Args... args) {
        return vector<decltype(make_v<T>(initvalue, args...))>(sz, make_v<T>(initvalue, args...));
    }

    template <class A, class B> inline bool chmax(A &a, const B &b) noexcept {
        return b > a && (a = b, true);
    }

    template <class A, class B> inline bool chmin(A &a, const B &b) noexcept {
        return b < a && (a = b, true);
    }

    template <class A, class B> inline common_type_t<A, B> min(const A &a, const B &b) noexcept {
        return a < b ? a : b;
    }

    template <class A, class B, class... Args>
    inline common_type_t<A, B, Args...> min(const A &a, const B &b, const Args&... args) noexcept {
        return moke::min(a < b ? a : b, args...);
    }

    template <class A, class B> inline common_type_t<A, B> max(const A &a, const B &b) noexcept {
        return a > b ? a : b;
    }

    template <class A, class B, class... Args>
    inline common_type_t<A, B, Args...> max(const A &a, const B &b, const Args&... args) noexcept {
        return moke::max(a > b ? a : b, args...);
    }

    template <class A, class B> inline common_type_t<A, B> diff(const A &a, const B &b) noexcept {
        return a < b ? b - a : a - b;
    }

} // namespace moke

// }}} End Header

namespace moke {

    int main_() {
        ios_base::sync_with_stdio(false);
        cin.tie(nullptr);

        var(string, s);
        var(size_t, K);

        set<string> S;
        const size_t N = s.length();

        rep(i, N) {
            for (size_t len = 1; i + len <= N && len <= K; ++len) {
                S.insert(s.substr(i, len));
            }
        }

        size_t i = 1;
        for (const auto e: S) {
            if (i == K) {
                return outl(e), 0;
            }
            ++i;
        }

        return 0;
    }

}

// {{{
signed main() {
    moke::main_();
    return 0;
}
// }}}


Measured memory_bytes: 1664