import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedWriter;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author anshh31
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        OutputWriter out = new OutputWriter(outputStream);
        TaskCKthSubstring solver = new TaskCKthSubstring();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskCKthSubstring {
        public void solve(int testNumber, InputReader in, OutputWriter out) {
            String s = in.next();
            int k = in.nextInt();
            String nextS = "";
            for (char i = 'a'; i <= 'z'; i++) {
                if (s.indexOf(i) != -1) {
                    nextS = i + "";
                    //out.println(nextS);
                    k--;
                } else continue;
                if (k == 0) break;
                for (char j = 'a'; j <= 'z'; j++) {
                    if (s.indexOf(nextS + j) != -1) {
                        nextS += j;
                        //out.println(nextS);
                        k--;
                    } else continue;
                    if (k == 0) break;
                    for (char l = 'a'; l <= 'z'; l++) {
                        if (s.indexOf(nextS + l) != -1) {
                            nextS += l;
                            //out.println(nextS);
                            k--;
                        } else continue;
                        if (k == 0) break;
                        for (char m = 'a'; m <= 'z'; m++) {
                            if (s.indexOf(nextS + m) != -1) {
                                nextS += m;
                                //out.println(nextS);
                                k--;
                            } else continue;
                            if (k == 0) break;
                            for (char n = 'a'; n <= 'z'; n++) {
                                if (s.indexOf(nextS + n) != -1) {
                                    nextS += n;
                                    //out.println(nextS);
                                    k--;
                                } else continue;
                                if (k == 0) break;
                                nextS = nextS.substring(0, nextS.length() - 1);
                            }
                            if (k == 0) break;
                            nextS = nextS.substring(0, nextS.length() - 1);
                        }
                        if (k == 0) break;
                        nextS = nextS.substring(0, nextS.length() - 1);
                    }
                    if (k == 0) break;
                    nextS = nextS.substring(0, nextS.length() - 1);
                }
                if (k == 0) break;
                nextS = nextS.substring(0, nextS.length() - 1);
            }
            out.println(nextS);
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (numChars == -1) {
                throw new InputMismatchException();
            }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }

        public int nextInt() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public String nextString() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            StringBuilder res = new StringBuilder();
            do {
                if (Character.isValidCodePoint(c)) {
                    res.appendCodePoint(c);
                }
                c = read();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        public boolean isSpaceChar(int c) {
            if (filter != null) {
                return filter.isSpaceChar(c);
            }
            return isWhitespace(c);
        }

        public static boolean isWhitespace(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public String next() {
            return nextString();
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);

        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void print(Object... objects) {
            for (int i = 0; i < objects.length; i++) {
                if (i != 0) {
                    writer.print(' ');
                }
                writer.print(objects[i]);
            }
        }

        public void println(Object... objects) {
            print(objects);
            writer.println();
        }

        public void close() {
            writer.close();
        }

    }
}



Measured memory_bytes: 23252