// compile: make data
// run: ./data < data.in
#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#ifdef LOCAL
#include <debug/codeforces.h>
#define debug(x...) _debug_print(#x, x);
#define Debug(x...) _debug_print_format(#x, x);
std::ifstream terminal("/dev/tty");
#define PP cerr<<"\033[1;30mpause...\e[0m",terminal.ignore();
#else
#define debug(x...)
#define Debug(x...)
#define PP
#endif
template<typename...Args> void print_(Args...args){((cout<<args<<" "),...)<<endl;}
#define VI vector<int>
#define VII vector<vector<int>>
#define VIII vector<vector<vector<int>>>
#define rep(i,a,b) for(int i=(a);i<(int)(b);++i)
#define sz(v) ((int)(v).size())
#define print(...) print_(__VA_ARGS__);
#define FIND(a, x) ((find(a.begin(),a.end(),(x))!=a.end())?1:0)
#define cmin(x,...) x=min({(x), __VA_ARGS__})
#define cmax(x,...) x=max({(x), __VA_ARGS__})
#define INTMAX (int)(9223372036854775807)
#define INF (int)(1152921504606846976)
#define NaN (int)(0x8b88e1d0595d51d1)
#define double long double
#define int long long
#define uint unsigned long long
#define MAXN 200010
 
struct sam {
    struct node {
        map<char, node*> next;
        node *link;
        int minlen, maxlen;
        int suf = 0;
        int endpos;
        int tot = 0, size = 0;
    };
    vector<node> datalist;
    int total = 0;
    node *last = nullptr, *root = nullptr;
    string s;
 
    node *newnode(int length = 0, node *p = nullptr) {
        node *x = &datalist[total++];
        x->minlen = 0;
        x->maxlen = length;
        x->link = p;
        return x;
    }
    sam(string str) {
        s = str;
        datalist.resize(s.size() * 2);
        root = last = newnode();
        root->endpos = 0;
        for (int i = 0; i < sz(s); ++i) {
            char c = s[i];
            node *cur = newnode(i+1, root);
            cur->endpos = i+1;
            cur->tot = 1;
            node *parent = last;
            while (parent && !parent->next.count(c)) {
                parent->next[c] = cur;
                parent = parent->link;
            }
            if (parent) {
                node *q = parent->next[c];
                if (parent->maxlen + 1 == q->maxlen) cur->link = q;
                else {
                    node *nq = newnode(parent->maxlen + 1, q->link);
                    nq->endpos = i+1;
                    nq->next = q->next;
                    q->link = cur->link = nq;
                    while (parent && parent->next[c] == q) {
                        parent->next[c] = nq;
                        parent = parent->link;
                    }
                }
            }
            last = cur;
        }
        // minlen
        queue<node*> q; q.push(root);
        while (!q.empty()) {
            node *x = q.front(); q.pop();
            for (auto [c, p]: x->next) {
                if (p->minlen) continue;
                p->minlen = x->minlen + 1;
                q.push(p);
            }
        }
        // is suffix
        for (node *x = last; x && x != root; x = x->link) x->suf = 1;
        // size of endpos set
        vector<vector<node*>> e(total);
        rep(i, 0, total) if (datalist[i].link) e[datalist[i].link - &datalist[0]].push_back(&datalist[i]);
        function<void(int)> dfs_tot = [&](int x) {
            for (auto y: e[x]) {
                dfs_tot(y - &datalist[0]);
                datalist[x].tot += datalist[y - &datalist[0]].tot;
            }
        };
        dfs_tot(0);
        // num of strings
        function<int(node*)> dfs_size = [&](node *x) {
            if (x->size) return x->size;
            x->size = x==root ? 0 : 1;
            for (auto [c, p]: x->next) x->size += dfs_size(p);
            return x->size;
        };
        dfs_size(root);
    }
    string getstr(node *x) {
        return s.substr(x->endpos - x->maxlen, x->maxlen);
    }
    void graphviz_dump(string filename = "graph.dot") {
        FILE *f = fopen(filename.c_str(), "w");
        fprintf(f, "digraph {\n");
        rep(i, 0, total) {
            node *x = &datalist[i];
            fprintf(f, "    %lld[%s shape=\"record\" label=\"", i, x->suf ? "color=\"red\"" : "");
            vector<string> desired = {
                "min=" + to_string(x->minlen),
                "max=" + to_string(x->maxlen),
                "endpos=" + to_string(x->endpos),
                "sz=" + to_string(x->size)
            };
            rep(j, 0, sz(desired)) {
                fprintf(f, "%s%s", j ? " | " : "", desired[j].c_str());
            }
            fprintf(f, "\"];\n");
        }
        rep(i, 0, total) {
            for (auto [c, p]: datalist[i].next) {
                fprintf(f, "    %lld -> %ld ;\n", i, p - &datalist[0], c);
            }
        }
        // rep(i, 0, total) if (datalist[i].link) fprintf(f, "    %lld -> %ld [color=\"blue\" style=\"dashed\"];\n", i, datalist[i].link - &datalist[0]);
        fprintf(f, "}\n");
    }
    void solve(int k) {
        node *x = root;
        for (; ; ) {
            if (!k) break;
            for (auto [c, p]: x->next) {
                if (k <= p->size) {
                    cout << c;
                    x = p;
                    --k;
                    break;
                }
                k -= p->size;
            }
        }
        cout << endl;
    }
};
 
int32_t main() {
    ios::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
 
    string s; cin >> s;
    int k; cin >> k;
    sam a(s);
    a.solve(k);
    // a.graphviz_dump();
 
    return 0;
}