433 3
FZDDHMJGBZCHFUXBBPIEBBEFDWOMXXEPOMDGSMPIUZOMRZQNSJAVNATGIWPDFISKFQXJNVFXPHOZDAEZFDAHDXXQKZMGNSGKQNWGNGJGJZVVITKNFLVCPMZSDMCHBTVAWYVZLIXXIADXNYILEYNIQHKMOGMVOCWGHCWIYMPEPADSJAAKEGTUSEDWAHMNYJDIHBKHVUHLYGNGZDBULRXLSAJHPCMNWCEAAPYMHDTYWPADOTJTXTXUKLCHWKUSZRHEKQEFPVJEJJHRWCKYOIWALRTIBUMNOCRXLSIKQCJVQXEPGOHRUDJDKMUUUDORURWXJNVRVMNOUNRFKSVMTMZGOIJLXEPAMVGESOADYIGZXRBJDIWKNOWTCSROAQTBECHTOZVSQUOOJRZIBAUHMKAXDCIMDZJFMABGRNTGPUJAUNFPFWCJG
 6 2
ABCABC
 41 19
TMEYYIIELFDCMBDKWWKYNRNDUPRONYROXQCLVQALP
 5 2
ABACA
 377 3
EADADBBBBDEAABBAEBABACDBDBBCACAADBEAEACDEAABACADEEDEACACDADABBBBDDEECBDABACACBAECBADAEBDEEBDBCDAEADBCDDACACDCCEEDBCCBBCEDBECBABCDDBBDEADEDAEACDECECBEBACBCCDCDBDAECDECADBCBEDBBDAAEBCAAECCDCCDBDDEBADEEBDCAEABBDEDBBDDEAECCBDDCDEACDAECCBDDABABEAEDCDEDBAECBDEACEBCECEACDCBABCBAAEAADACADBBBBABEADBCADEBCBECCABBDDDEEBCDEBADEBDAAABBEABADEDEAEABCEEBEEDEAEBEABCEDDBACBCCADEBAAAAAEABABBCE
 3 26
AAB
 27 1
ABCDEFGHIJKLMNOPQRSTUVWXYZA
 73 2
DEBECECBBADAADEAABEAEEEAEBEAEBCDDBABBAEBACCBEEBBAEADEECACEDEEDABACDCDBBBD
 7 2
ABCDBCD
 5 1
AZAZA
 44 15
HGJIFCGGCDGIJDHBIBGAEABCIABIGBDEADBBBAGDFDHA
 5 2
ABCAB
 8 3
ABCBCDCA
 26 1
ABCDEFGHIJKLMNOPQRSTUVWXYZ

YES
 YES
 NO
 NO
 YES
 NO
 YES
 YES
 YES
 YES
 NO
 YES
 NO
 NO


import java.util.*;
import java.io.*;
import java.math.*;
public class Codechef{
    static FR in;
    public static void main(String[] args)throws Exception{
    	in =new FR();
    	
    	int n=ni(), k=ni();
    	String s =n();
    	int[] pref =new int[26];
    	int[] suff =new int[26];
    	for( int i=0;i<26;i++ ){ pref[i]=-1; suff[i]=-1; }
    	for( int i=s.length()-1;i>=0;i-- )
    		if( suff[(int)(s.charAt(i)-'A')]==-1 )suff[(int)(s.charAt(i)-'A')]=i;
    	for( int i=0;i<s.length();i++ )
    		if( pref[(int)(s.charAt(i)-'A')]==-1 )pref[(int)(s.charAt(i)-'A')]=i;
    	
    	int add[]=new int[s.length()+1];
    	for( int i=0;i<26;i++ ){
    		if( pref[i]!=-1 ){ add[pref[i]]++; add[suff[i]+1]--; }
    	}
    	for( int i=1;i<s.length();i++ ){
    		add[i]+=add[i-1];
    	}
    	int max=0;
    	for( int i=0;i<s.length();i++ )max=Math.max(max, add[i]);
    	pn(max<=k ? "No": "Yes");
    }   
    
    static long[]la( int N ){
    	long[]a=new long[N];
    	for( int i=0;i<N;i++ )a[i]=nl();
    	return a;
    }
    
    static int[] ia(int N){
        int[] a = new int[N+1];
        for(int i = 1; i<=N; i++)a[i] = ni();
        return a;
    }
    
    static class FR{
        BufferedReader br;
        StringTokenizer st;
        public FR(){
            br = new BufferedReader(new InputStreamReader(System.in));
        }
 
        String next(){
            while (st == null || !st.hasMoreElements()){
                try{
                    st = new StringTokenizer(br.readLine());
                }catch (IOException  e){
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }
 
        String nextLine(){
            String str = "";
            try{
                str = br.readLine();
            }catch (IOException e){
                e.printStackTrace();
            }
            return str;
        }
    }
 
    static void p(Object o){
        System.out.print(o);
    }
    
    static void pn(Object o){
        System.out.println(o);
    }
    
    static String n(){
        return in.next();
    }
    
    static String nln(){
        return in.nextLine();
    }
    
    static int ni(){
        return Integer.parseInt(in.next());
    }
    
    static long nl(){
        return Long.parseLong(in.next());
    }
} 