import java.io.InputStream;
import java.io.IOException;
import java.io.PrintStream;
import java.util.*;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main {

  public static void main(String[] args) {
    solve(System.in, System.out);
  }

  static void solve(InputStream is, PrintStream os) {
    Scanner sc = new Scanner(is);

    /* read */
    String s = sc.next();
    int k = sc.nextInt();

    Queue<String> queue = new PriorityQueue<>(Comparator.reverseOrder());

    Set<String> visited = new HashSet<>(s.length() * s.length());

    for (int i = 0; i < s.length(); i++) {
      int j = i + 1;
      while (j <= Math.min(s.length(), i + k)) {
        String sub = s.substring(i, j);
        j++;
        if (visited.contains(sub)) {
          continue;
        }
        visited.add(sub);
        if (queue.size() < k) {
          queue.add(sub);
          continue;
        }
        String top = queue.peek();
        if (top.compareTo(sub) < 0) {
          // further substring is more larger, so no need to check;
          break;
        }
        queue.remove();
        queue.add(sub);
      }
    }
    os.println(queue.remove());
  }

  private static <P> List<P> bfs(P start, P end, Function<P, Set<P>> travel,
      Predicate<P> predicate) {
    Set<P> visited = new HashSet<>();
    Queue<List<P>> queue = new LinkedList<>();
    visited.add(start);
    queue.add(Collections.singletonList(start));

    while (!queue.isEmpty()) {
      List<P> path = queue.remove();
      P head = path.get(path.size() - 1);
      Set<P> candidates = travel.apply(head);
      for (P c : candidates) {
        if (!visited.contains(c) && predicate.test(c)) {
          List<P> p = Stream.concat(path.stream(), Stream.of(c)).collect(Collectors.toList());
          queue.add(p);
          visited.add(c);
          if (c.equals(end)) {
            return p;
          }
        }
      }
    }
    return null;
  }

  private static class FermatCombination {

    private long fact[];
    private long mod;

    public FermatCombination(int size, long mod) {
      this.fact = new long[size + 1];
      this.mod = mod;

      this.fact[0] = 1;

      for (int i = 1; i <= size; i++) {
        fact[i] = (fact[i - 1] * i) % mod;
      }
    }

    private long factorial(int n) {
      return fact[n];
    }

    private long inverse(long n) {
      return pow(n, mod - 2) % mod;
    }

    private long pow(long x, long n) {
      long ans = 1;
      while (n > 0) {
        if ((n & 1) == 1) {
          ans = ans * x % mod;
        }
        x = x * x % mod;
        n >>= 1;
      }
      return ans;
    }

    long combination(int n, int k) {
      long ans = 1;
      ans *= factorial(n);
      ans %= mod;
      ans *= inverse(factorial(n - k));
      ans %= mod;
      ans *= inverse(factorial(k));
      ans %= mod;
      return ans;
    }
  }

  private static class Scanner {

    private final InputStream is;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;

    Scanner(InputStream is) {
      this.is = is;
    }

    private boolean hasNextByte() {
      if (ptr < buflen) {
        return true;
      } else {
        ptr = 0;
        try {
          buflen = is.read(buffer);
        } catch (IOException e) {
          e.printStackTrace();
        }
        if (buflen <= 0) {
          return false;
        }
      }
      return true;
    }

    private int readByte() {
      if (hasNextByte()) {
        return buffer[ptr++];
      } else {
        return -1;
      }
    }

    private static boolean isPrintableChar(int c) {
      return 33 <= c && c <= 126;
    }

    public boolean hasNext() {
      while (hasNextByte() && !isPrintableChar(buffer[ptr])) {
        ptr++;
      }
      return hasNextByte();
    }

    public String next() {
      if (!hasNext()) {
        throw new NoSuchElementException();
      }
      StringBuilder sb = new StringBuilder();
      int b = readByte();
      while (isPrintableChar(b)) {
        sb.appendCodePoint(b);
        b = readByte();
      }
      return sb.toString();
    }

    public long nextLong() {
      if (!hasNext()) {
        throw new NoSuchElementException();
      }
      long n = 0;
      boolean minus = false;
      int b = readByte();
      if (b == '-') {
        minus = true;
        b = readByte();
      }
      if (b < '0' || '9' < b) {
        throw new NumberFormatException();
      }
      while (true) {
        if ('0' <= b && b <= '9') {
          n *= 10;
          n += b - '0';
        } else if (b == -1 || !isPrintableChar(b)) {
          return minus ? -n : n;
        } else {
          throw new NumberFormatException();
        }
        b = readByte();
      }
    }

    public int nextInt() {
      long nl = nextLong();
      if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) {
        throw new NumberFormatException();
      }
      return (int) nl;
    }

    public double nextDouble() {
      return Double.parseDouble(next());
    }
  }
}


Measured memory_bytes: 155348