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 loser
{
    static class InputReader {
        public BufferedReader br;
        public StringTokenizer token;
        public InputReader(InputStream stream)
        {
            br=new BufferedReader(new InputStreamReader(stream),32768);
            token=null;
        }

        public String next()
        {
            while(token==null || !token.hasMoreTokens())
            {
                try
                {
                    token=new StringTokenizer(br.readLine());
                }
                catch(IOException e)
                {
                    throw new RuntimeException(e);
                }
            }
            return token.nextToken();
        }

        public int nextInt()
        {
            return Integer.parseInt(next());
        }

        public long nextLong()
        {
            return Long.parseLong(next());
        }
    }
    static class card{
        long a;
        int i;
        public card(long a,int i)
        {
            this.a=a;
            this.i=i;
        }
    }
    static class sort implements Comparator<pair>
    {
        public int compare(pair o1,pair o2)
        {
            if(o1.a!=o2.a)
                return (int)(o1.a-o2.a)*-1;
            else
                return (int)(o1.b-o2.b)*-1;
        }
    }
    static class sort1 implements Comparator<pair>
    {
        public int compare(pair o1,pair o2)
        {
            if(o1.b!=o2.b)
                return (int)(o1.b-o2.b);
            else
                return -1;
        }
    }
    static void shuffle(int a[])
    {
        List<Integer> l=new ArrayList<>();
        for(int i=0;i<a.length;i++)
            l.add(a[i]);
        Collections.shuffle(l);
        for(int i=0;i<a.length;i++)
            a[i]=l.get(i);
    }

    /*static long gcd(long a,long b)
    {
    if(b==0)
    return a;
    else
    return gcd(b,a%b);
    }*/
    /*static boolean valid(int i,int j)
    {
    if(i<4 && i>=0 && j<4 && j>=0)
    return true;
    else
    return false;
    }*/
    static class pair{
        long a;int b;
        public pair(long a,int b)
        {
            this.a=a;
            this.b=b;
        }
    }
    static boolean valid(int i,int j,int r,int c)
    {
        if(i>=0 && i<r && j>=0 && j<c)
            return true;
        else
            return false;
    }

    public  static void main(String[] args) 
    {
        InputReader sc=new InputReader(System.in);
        int n=sc.nextInt();
        int k=sc.nextInt();
        char c[]=new char[n];
        c=sc.next().toCharArray();
        List<Integer> s=new ArrayList<>();
        List<Integer> e=new ArrayList<>();
        int a[]=new int[100];
        int c1=0;
        for(int i=0;i<n;i++)
        {
            if(a[c[i]]==0)
            {
                a[c[i]]=1;
                c1++;
                s.add(i);
                for(int j=n-1;j>=0;j--)
                {
                    if(c[j]==c[i])
                    {
                        e.add(j);
                        break;
                    }
                }
            }
        }
        Collections.sort(e);
        int j=0,p=0;
        for(int i=0;i<c1;i++)
        {
            if(k>0)
            {
                k--;
            }
            else
            {
                int t=s.get(i);
                for(j=p;j<c1;j++)
                {
                    if(t>e.get(j))
                    k++;
                    else
                    break;
                }
                if(k==0)
                {
                    System.out.println("YES");
                    System.exit(0);
                }
                p=j;
                k--;
            }
        }
        System.out.println("NO");
    }
}
