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.*;
import java.util.*;
import java.math.*;

/**
 *
 * @author ghost
 */
public class B_834 {

 
    public static void main(String args[])
    {
 
        //File file= new File("COMPLETE_FILE_PATH");
        //InputStream is = new FileInputStream(file);
        
        
        //InputReader in = new InputReader(is);
        InputReader in = new InputReader(System.in);
        PrintWriter out= new PrintWriter(System.out);
        
        int t=1;
        
        Task solver=new Task();
        solver.solve(t,in,out);
        
 
        out.flush();
        out.close();
 
 
    }
 
    static class Task {
        long mod = 1000000007;
 
        public void solve(int testNumber, InputReader in,PrintWriter out){
 
            int n=in.nextInt();int  k=in.nextInt();
            
            char[] guest=new char[n];
            guest=in.next().toCharArray();
            
            int[][] timing=new int[26][2];
            for(int i=0;i<26;i++){
                timing[i][1]=Integer.MIN_VALUE;
                timing[i][0]=Integer.MAX_VALUE;
           }
            
            for(int i=0;i<guest.length;i++){
                if(timing[guest[i]-'A'][0]>i)  {
                    timing[guest[i]-'A'][0]=i;
                    
                }
                if(timing[guest[i]-'A'][1]<i){
                    timing[guest[i]-'A'][1]=i;
                }
            } 
            
//            for(int i=0;i<26;i++){
//                
//                    if(timing[i][0]>1e6 ) timing[i][0]=0;
//                    if(timing[i][1]<0)    timing[i][1]=0;
//            }
            
//            for(int i=0;i<26;i++){
//                for(int j=0;j<2;j++){
//                    out.print(timing[i][j]+" ");
//                }
//                out.println();
//            }

            int count=0;
            int req=0;
            for(int i=0;i<n;i++){
                if(i==timing[guest[i]-'A'][0])
                    req++;
                count=Math.max(count,req);
                if(i==timing[guest[i]-'A'][1])
                    req--;
                
                
            }
            out.println(k>=count?"NO":"YES");
        }
            
        int LOG2(long item){
            int count=0;
            while(item>1){
                item>>=1;count++;
            }
            return count;
        }
 
 
        long gcd(long a, long b) {
            a = Math.abs(a);
            b = Math.abs(b);
            return BigInteger.valueOf(a).gcd(BigInteger.valueOf(b)).longValue();
        }
    }
    
    
    static class InputReader {
 
        private InputStream stream;
        private byte[] buf = new byte[8192];
        private int curChar, snumChars;
        private SpaceCharFilter filter;
 
        public InputReader(InputStream stream) {
            this.stream = stream;
        }
 
        public int snext() {
            if (snumChars == -1)
                throw new InputMismatchException();
            if (curChar >= snumChars) {
                curChar = 0;
                try {
                    snumChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (snumChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }
 
        public int nextInt() {
            int c = snext();
            while (isSpaceChar(c))
                c = snext();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = snext();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = snext();
            } while (!isSpaceChar(c));
            return res * sgn;
        }
 
        public long nextLong() {
            int c = snext();
            while (isSpaceChar(c))
                c = snext();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = snext();
            }
            long res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = snext();
            } while (!isSpaceChar(c));
            return res * sgn;
        }
 
        public int[] nextIntArray(int n) {
            int a[] = new int[n];
            for (int i = 0; i < n; i++)
                a[i] = nextInt();
            return a;
        }
 
        public String readString() {
            int c = snext();
            while (isSpaceChar(c))
                c = snext();
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = snext();
            } while (!isSpaceChar(c));
            return res.toString();
        }
 
        public String next() {
            return readString();
        }
 
        public boolean isSpaceChar(int c) {
            if (filter != null)
                return filter.isSpaceChar(c);
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }
 
        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);
        }

        private static void pa(Object... o) {
            System.out.println(Arrays.deepToString(o));
        }
    }

}