import std.algorithm;
import std.array;
import std.ascii;
import std.bigint;
import std.complex;
import std.container;
import std.conv;
import std.functional;
import std.math;
import std.range;
import std.stdio;
import std.string;
import std.typecons;

auto readInts() {
	return array(map!(to!int)(readln().strip().split()));
}
auto readInt() {
	return readInts()[0];
}
auto readLongs() {
	return array(map!(to!long)(readln().strip().split()));
}
auto readLong() {
	return readLongs()[0];
}

void readlnTo(T...)(ref T t) {
    auto s = readln().split();
    assert(s.length == t.length);
    foreach(ref ti; t) {
        ti = s[0].to!(typeof(ti));
        s = s[1..$];
    }
}

class UnionFindTree {
public:
    this(size_t n) {
        parent = iota(n).array();
        rank = new size_t[](n);
    }
    size_t find(size_t n) {
        if(n == parent[n]) {
            return n;
        } else {
            parent[n] = find(parent[n]);
            return parent[n];
        }
    }
    void unite(size_t n, size_t m) {
        auto p1 = find(n);
        auto p2 = find(m);
        if(n == m) {
            return;
        }
        if(rank[p1] < rank[p2]) {
            swap(p1, p2);
        }
        if(rank[p1] == rank[p2]) {
            ++rank[p1];
        }
        parent[p2] = p1;
    }
private:
    size_t[] parent;
    size_t[] rank;
}
unittest {
    auto uft = new UnionFindTree(10);
    foreach(i; iota(10)) {
        assert(uft.find(i) == i);
    }
    foreach(i; iota(10).filter!(a => a%3 == 0).drop(1)) {
        uft.unite(i, i-3);
    }
    foreach(i; iota(10).filter!(a => a%3 == 0)) {
        assert(uft.find(0) == uft.find(i));
    }
    foreach(i; iota(10).filter!(a => a%3 != 0)) {
        assert(uft.find(0) != uft.find(i));
    }
}

//内部では1-based, 使うときは0-based
class BinaryIndexedTree(T, alias F = "a + b") if(is(typeof(binaryFun!F))) {
    alias f = binaryFun!F;
public:
    this(in size_t n, in T T0 = T.init) {
        t.length = n+1;
        t[] = T0;
    }

    void add(in size_t idx, in T v)
    in {
        assert(idx < t.length-1);
    }
    body {
        one_based_add(idx+1, v);
    }

    T accumulate(size_t idx) {
        return one_based_accumulate(idx+1);
    }

private:
    T[] t;

    //1-based
    void one_based_add(size_t idx, in ref T v) {
        if(idx >= t.length) {
            return;
        }
        t[idx] = f(t[idx], v);
        one_based_add(idx + (idx & (-idx)), v);
    }
    T one_based_accumulate(size_t idx) {
        if(idx == 0) {
            return t[0];
        } else {
            return f(one_based_accumulate(idx - (idx & (-idx))), t[idx]);
        }
    }
}
unittest {
    auto bit = new BinaryIndexedTree!(long)(10);
    foreach(i; iota(10)) {
        assert(bit.accumulate(i) == 0);
    }
    bit.add(5, 3);
    foreach(i; iota(5)) {
        assert(bit.accumulate(i) == 0);
    }
    foreach(i; iota(5, 10)) {
        assert(bit.accumulate(i) == 3);
    }

    bit.add(7, 2);

    foreach(i; iota(5)) {
        assert(bit.accumulate(i) == 0);
    }
    foreach(i; iota(5, 7)) {
        assert(bit.accumulate(i) == 3);
    }
    foreach(i; iota(7, 10)) {
        assert(bit.accumulate(i) == 5);
    }
}
unittest {
    auto bit = new BinaryIndexedTree!(long, "a*b")(10, 1);
    foreach(i; iota(10)) {
        assert(bit.accumulate(i) == 1);
    }
    bit.add(5, 3);
    foreach(i; iota(5)) {
        assert(bit.accumulate(i) == 1);
    }
    foreach(i; iota(5, 10)) {
        assert(bit.accumulate(i) == 3);
    }

    bit.add(7, 2);

    foreach(i; iota(5)) {
        assert(bit.accumulate(i) == 1);
    }
    foreach(i; iota(5, 7)) {
        assert(bit.accumulate(i) == 3);
    }
    foreach(i; iota(7, 10)) {
        assert(bit.accumulate(i) == 6);
    }
}

//最大流、O(|V||E|^2)
template Flow_Dinic() {
    class Edge {
    public:
        this(long to, long capacity) {
            this.to = to;
            this.capacity = capacity;
        }
        long to;
        long capacity;
        Edge reverse;
    }

    //gを残余グラフにする
    void residualise(Edge[][] g) {
        foreach(u; iota(g.length)) {
            foreach(e; g[u]) {
                if(e.capacity != 0) {
                    e.reverse = new Edge(u, 0);
                    e.reverse.reverse = e;
                    g[e.to] ~= e.reverse;
                }
            }
        }
    }

    const long INF = long.max / 2;

    //gは残余グラフ
    //gは計算の結果書き換わる
    long solve(Edge[][] g, long s, long t) {
        long ret;
        while(true) {
            auto level = new long[](g.length);
            level[] = INF;
            auto q = DList!long(s);
            level[s] = 0;
            //level graph
            auto lg = new Edge[][](g.length, 0);
            while(!q.empty()) {
                auto u = q.front();
                q.removeFront();
                if(u == t) {
                    break;
                }
                foreach(e; g[u]) {
                    if(e.capacity > 0 && level[e.to] >= level[u] + 1) {
                        level[e.to] = level[u] + 1;
                        q.insertBack(e.to);
                        lg[u] ~= e;
                    }
                }
            }
            if(level[t] == INF) {
                break;
            }
            auto iter = new long[](g.length);
            long dfs(long u, long c_min) {
                if(u == t) {
                    return c_min;
                }
                foreach(i; iota(iter[u], lg[u].length)) {
                    iter[u] = i;
                    long f = dfs(lg[u][i].to, min(c_min, lg[u][i].capacity));
                    if(f > 0) {
                        lg[u][i].capacity -= f;
                        lg[u][i].reverse.capacity += f;
                        return f;
                    }
                }
                return 0;
            }
            while(true) {
                long f = dfs(s, INF);
                if(f == 0) {
                    break;
                }
                ret += f;
            }
        }
        return ret;
    }
}

enum real eps = 1e-10;

enum long mod = 1_000_000_000 + 7;

void main(){
  auto s = readln().chomp();
  auto k = readLong();
  string[] t;
  foreach(i; iota(s.length)){
    foreach(j; iota(1, k+1)) {
      if (i + j <= s.length){ 
        t ~= s[i..i+j];
      }
    }
  }
  t.sort();
  writeln(t.uniq.array[k-1]);
}




Measured memory_bytes: 2812