using System;
using System.Diagnostics;
using System.IO;
using System.Text;
using System.Linq;
using System.Collections.Generic;
using System.Text.RegularExpressions;

class Program
{
    const string Yes = "Yes";
    const string No = "No";
    const long Mod = 1000000007;

    static void Main(string[] args) {
#if DEBUG
        MyDebugger.Test(process, "../TextFile1.txt");
#else
        object ret = process(new StreamReader(Console.OpenStandardInput()));
        Console.WriteLine(ret);
#endif
    }

    static object process(TextReader input) {
        var s = input.ReadLine();
        var k = int.Parse(input.ReadLine());
        var n = s.Length;

        var q = new PriorityQueue<C>(n*n);

        for(int i=0; i<n; i++) {
            var c = new C() { str = s[i].ToString(), idx=i };
            q.Push(c);
        }

        var hash = new HashSet<string>();

        while (true) {
            var c = q.Pop();

            if (!hash.Contains(c.str)) {
                hash.Add(c.str);
                if (hash.Count >= k) return c.str;
            }

            if(c.idx < n - 1) {
                c.str += s[c.idx + 1];
                c.idx = c.idx + 1;
                q.Push(c);
            }
        }
    }

    class C : IComparable
    {
        public string str;
        public int idx;

        public int CompareTo(object obj) {
            return this.str.CompareTo((obj as C).str);
        }
    }

    static int[] ToIntArray(string strs, int n) {
        var ret = new int[n];
        var spl = strs.Split(' ');
        for (int i = 0; i < n; i++)
            ret[i] = int.Parse(spl[i]);
        return ret;
    }

    //コピペ
    public class PriorityQueue<T> where T : IComparable
    {
        private IComparer<T> _comparer = null;
        private int _type = 0;

        private T[] _heap;
        private int _sz = 0;

        private int _count = 0;

        /// <summary>
        /// Priority Queue with custom comparer
        /// </summary>
        public PriorityQueue(int maxSize, IComparer<T> comparer) {
            _heap = new T[maxSize];
            _comparer = comparer;
        }

        /// <summary>
        /// Priority queue
        /// </summary>
        /// <param name="maxSize">max size</param>
        /// <param name="type">0: asc, 1:desc</param>
        public PriorityQueue(int maxSize, int type = 0) {
            _heap = new T[maxSize];
            _type = type;
        }

        private int Compare(T x, T y) {
            if (_comparer != null) return _comparer.Compare(x, y);
            return _type == 0 ? x.CompareTo(y) : y.CompareTo(x);
        }

        public void Push(T x) {
            _count++;

            //node number
            var i = _sz++;

            while (i > 0) {
                //parent node number
                var p = (i - 1) / 2;

                if (Compare(_heap[p], x) <= 0) break;

                _heap[i] = _heap[p];
                i = p;
            }

            _heap[i] = x;
        }

        public T Pop() {
            _count--;

            T ret = _heap[0];
            T x = _heap[--_sz];

            int i = 0;
            while (i * 2 + 1 < _sz) {
                //children
                int a = i * 2 + 1;
                int b = i * 2 + 2;

                if (b < _sz && Compare(_heap[b], _heap[a]) < 0) a = b;

                if (Compare(_heap[a], x) >= 0) break;

                _heap[i] = _heap[a];
                i = a;
            }

            _heap[i] = x;

            return ret;
        }

        public int Count() {
            return _count;
        }

        public T Peek() {
            return _heap[0];
        }

        public bool Contains(T x) {
            for (int i = 0; i < _sz; i++) if (x.Equals(_heap[i])) return true;
            return false;
        }

        public void Clear() {
            while (this.Count() > 0) this.Pop();
        }

        public IEnumerator<T> GetEnumerator() {
            var ret = new List<T>();

            while (this.Count() > 0) {
                ret.Add(this.Pop());
            }

            foreach (var r in ret) {
                this.Push(r);
                yield return r;
            }
        }

        public T[] ToArray() {
            T[] array = new T[_sz];
            int i = 0;

            foreach (var r in this) {
                array[i++] = r;
            }

            return array;
        }
    }
}


Measured memory_bytes: 13664