import java.util.Iterator;
import java.util.PrimitiveIterator;
import java.util.Scanner;
import java.util.TreeSet;
import java.util.function.Supplier;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class Main{
	static Scanner s=new Scanner(System.in);

	static String	v	=s.next();
	static int		n	=v.length();
	static class StringView implements Comparable<StringView>{
		int begin,end;
		public StringView(int begin,int end){
			this.begin=begin;
			this.end=end;
		}
		
		@Override
		public int hashCode(){
			final int prime=31;
			int result=1;
			result=prime*result+begin;
			result=prime*result+end;
			return result;
		}
		@Override
		public boolean equals(Object obj){
			if(this==obj) return true;
			if(obj==null) return false;
			if(getClass()!=obj.getClass()) return false;
			StringView other=(StringView)obj;
			if(begin!=other.begin) return false;
			if(end!=other.end) return false;
			return true;
		}

		@Override
		public int compareTo(StringView o){
			for(int i=0;true;++i) {
				int a=begin+i<end?v.charAt(begin+i):-1;
				int b=o.begin+i<o.end?v.charAt(o.begin+i):-1;
				int c=Integer.compare(a,b);
				if(c!=0||(a==-1&&b==-1))
					return c;
			}
		}
		
		@Override
		public String toString(){
			return v.substring(begin,end);
		}
	}
	void solve(){
		TreeSet<StringView> l=new TreeSet<>();
		int k=gInt();
		for(int i:rep(n)){
			for(int j:rep(i+1,n)){
				l.add(new StringView(i,j));
				if(l.size()>k)
					l.pollLast();
			}
		}
		System.out.println(l.last());
	}
	public static void main(String[] A){
		new Main().solve();
	}

	static int gInt(){
		return Integer.parseInt(s.next());
	}
	static long gLong(){
		return Long.parseLong(s.next());
	}
	static double gDouble(){
		return Double.parseDouble(s.next());
	}

	SupplyingIterator<Integer> ints(int n){
		return new SupplyingIterator<>(n,Main::gInt);
	}
	SupplyingIterator<Long> longs(int n){
		return new SupplyingIterator<>(n,Main::gLong);
	}
	SupplyingIterator<Double> doubles(int n){
		return new SupplyingIterator<>(n,Main::gDouble);
	}
	SupplyingIterator<String> strs(int n){
		return new SupplyingIterator<>(n,s::next);
	}

	Range rep(int i){
		return Range.rep(i);
	}
	Range rep(int f,int t,int d){
		return Range.rep(f,t,d);
	}
	Range rep(int f,int t){
		return rep(f,t,1);
	}
	Range rrep(int f,int t){
		return rep(t,f,-1);
	}

	IntStream REP(int v){
		return IntStream.range(0,v);
	}
	IntStream REP(int l,int r){
		return IntStream.rangeClosed(l,r);
	}

	IntStream INTS(int n){
		return IntStream.generate(Main::gInt).limit(n);
	}
	Stream<String> STRS(int n){
		return Stream.generate(s::next).limit(n);
	}

}
class SupplyingIterator<T> implements Iterable<T>,Iterator<T>{
	int			t;
	Supplier<T>	supplier;

	SupplyingIterator(int t,Supplier<T> supplier){
		this.t=t;
		this.supplier=supplier;
	}

	@Override
	public Iterator<T> iterator(){
		return this;
	}

	@Override
	public boolean hasNext(){
		return t>0;
	}

	@Override
	public T next(){
		--t;
		return supplier.get();
	}

}
class Range implements Iterable<Integer>,PrimitiveIterator.OfInt{
	int to,cur,d;

	Range(int from,int to,int d){
		this.cur=from-d;
		this.to=to;
		this.d=d;
	}

	Range(int n){
		this(0,n-1,1);
	}

	@Override
	public Iterator<Integer> iterator(){
		return this;
	}

	@Override
	public boolean hasNext(){
		return cur+d==to||(cur!=to&&(cur<to==cur+d<to));
	}

	@Override
	public int nextInt(){
		return cur+=d;
	}

	static Range rep(int i){
		return new Range(i);
	}
	static Range rep(int f,int t,int d){
		return new Range(f,t,d);
	}
	static Range rep(int f,int t){
		return rep(f,t,1);
	}
	static Range rrep(int f,int t){
		return rep(f,t,-1);
	}

}


Measured memory_bytes: 224404