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.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.*;

public class FestiveEvening {
    Node[] arr = new Node[26];
    static class Node{
        int l = -1;
        int r = -1;
        boolean stat = false;
        Node(int l, int r){
            this.l = l;
            this.r = r;
        }
    }
    public void insert(int x, int i){
        if(arr[x]==null){
            Node n = new Node(i,-1);
            arr[x] = n;
        }
        else{
            arr[x].r = i;
        }
    }
    public int checkActive(int day){
        int count = 0;
        for(int i=0; i<arr.length; i++){
            if(arr[i]!=null){
                if(arr[i].l==day) {
                    arr[i].stat = true;
                }
                if(arr[i].r==day-1){
                    arr[i].stat = false;
                }
                if(arr[i].stat==true){
                    count++;
                }
            }
        }
        return count;
    }
    public static void main(String[] args) throws IOException{
        Reader.init(System.in);
        int n = Reader.nextInt();
        int k = Reader.nextInt();
        FestiveEvening h = new FestiveEvening();
        String s = Reader.next();
        for(int i=0; i<n; i++){
            char c = s.charAt(i);
            int x = (int)c - 65;
            h.insert(x,i);
        }
        for(int i=0; i<26; i++){
            if(h.arr[i] != null && h.arr[i].r==-1){
                h.arr[i].r = h.arr[i].l;
            }
        }
        int activeCount = 0;
        int sg = 0;
        for(int i=0; i<=n; i++){
            activeCount = h.checkActive(i);
            if(activeCount>k){
                sg = 1;
            }
        }
        if(sg==1){
            System.out.println("YES");
        }
        else{
            System.out.println("NO");
        }
    }
}
class Reader {
    static BufferedReader reader;
    static StringTokenizer tokenizer;

    static void init(InputStream input) {
        reader = new BufferedReader(
                new InputStreamReader(input) );
        tokenizer = new StringTokenizer("");
    }

    static String next() throws IOException {
        while ( ! tokenizer.hasMoreTokens() ) {
            tokenizer = new StringTokenizer(
                    reader.readLine() );
        }
        return tokenizer.nextToken();
    }

    static int nextInt() throws IOException {
        return Integer.parseInt( next() );
    }

    static double nextDouble() throws IOException {
        return Double.parseDouble( next() );
    }

    static long nextLong() throws IOException {
        return Long.parseLong( next() );
    }
}
