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.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Random;
import java.util.StringTokenizer;

public class B {
    public static void main (String[] args) {new B();}

    B() {
        FastScanner fs = new FastScanner();
        PrintWriter out = new PrintWriter(System.out);

        int n = fs.nextInt(), k = fs.nextInt();
        char[] a = fs.nextCharArray();

        int[] first = new int[26];
        int[] last = new int[26];
        Arrays.fill(first, -1);

        for(int i = 0; i < n; i++) {
            int idx = a[i] - 'A';
            if(first[idx] < 0) {
                first[idx] = i;
            }
            last[idx] = i;
        }


        boolean res = false;
        int hold = k;

        int[] mark = new int[n + 1];
        for(int i = 0; i < 26; i++) {
            if(first[i] < 0) continue;
            mark[first[i]]++;
            mark[last[i] + 1]--;
        }

        int sum = 0;
        for(int i = 0; i < mark.length; i++) {
            sum += mark[i];
            if(sum > k) {
                res = true;
            }
        }

        out.println(res ? "YES" : "NO");


        out.close();
    }

    void sort (int[] a) {
        int n = a.length;
        for(int i = 0; i < 1000; i++) {
            Random r = new Random();
            int x = r.nextInt(n), y = r.nextInt(n);
            int temp = a[x];
            a[x] = a[y];
            a[y] = temp;
        }
        Arrays.sort(a);
    }

    class FastScanner {
        BufferedReader br;
        StringTokenizer st;
        public FastScanner() {
            try	{
                br = new BufferedReader(new InputStreamReader(System.in));
//                br = new BufferedReader(new FileReader("testdata.out"));
                st = new StringTokenizer("");
            } catch (Exception e){e.printStackTrace();}
        }
        public String next() {
            if (st.hasMoreTokens())	return st.nextToken();
            try {st = new StringTokenizer(br.readLine());}
            catch (Exception e) {e.printStackTrace();}
            return st.nextToken();
        }
        public int nextInt() {return Integer.parseInt(next());}
        public long nextLong() {return Long.parseLong(next());}
        public double nextDouble() {return Double.parseDouble(next());}

        public String nextLine() {
            String line = "";
            try {line = br.readLine();}
            catch (Exception e) {e.printStackTrace();}
            return line;
        }
        public Integer[] nextIntegerArray(int n) {
            Integer[] a = new Integer[n];
            for(int i = 0; i < n; i++) a[i] = nextInt();
            return a;
        }
        public int[] nextIntArray(int n) {
            int[] a = new int[n];
            for(int i = 0; i < n; i++) a[i] = nextInt();
            return a;
        }
        public char[] nextCharArray() {return nextLine().toCharArray();}
    }

}
