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.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.util.Collections;
import java.util.ArrayList;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author ankur
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        sdavh solver = new sdavh();
        solver.solve(1, in, out);
        out.close();
    }

    static class sdavh {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int n = in.nextInt();
            int k = in.nextInt();
            String str = in.readString();

            // System.out.println((int)'A');
            int ar[] = new int[26];
            int ar2[] = new int[26];
            for (int i = 0; i < 26; i++) {
                ar[i] = -1;
                ar2[i] = (int) 1e7;
            }
            for (int i = 0; i < str.length(); i++) {
                ar[str.charAt(i) - 'A'] = Math.max(i + 1, ar[str.charAt(i) - 'A']);
            }
            for (int i = 0; i < str.length(); i++) {
                ar2[str.charAt(i) - 'A'] = Math.min(i + 1, ar2[str.charAt(i) - 'A']);
            }

            ArrayList<pair> ans = new ArrayList<>();
            for (int i = 0; i < 26; i++) {
                if (ar[i] != -1) {
                    pair temp = new pair();
                    temp.s = 1;
                    temp.time = ar[i];
                    ans.add(temp);
                }
                if (ar2[i] < 1e7) {
                    pair temp = new pair();
                    temp.s = 0;
                    temp.time = ar2[i];
                    ans.add(temp);
                }
            }
            Collections.sort(ans);
            int max = -1;
            int ct = 0;
            for (int i = 0; i < ans.size(); i++) {
                if (ans.get(i).s == 0) {
                    ct++;
                } else
                    ct--;
                max = Math.max(max, ct);
            }
            //out.print(ans.size());
            // out.print(max);
            if (k < max) {
                out.print("YES");
            } else {
                out.print("NO");
            }


        }

        class pair implements Comparable<pair> {
            int s;
            int time;

            public int compareTo(pair o) {
                if (this.time == o.time) {
                    return -this.s + o.s;
                } else
                    return this.time - o.time;
            }

        }

    }

    static class InputReader {
        private final InputStream stream;
        private final byte[] buf = new byte[8192];
        private int curChar;
        private int snumChars;

        public InputReader(InputStream st) {
            this.stream = st;
        }

        public int read() {
            //*-*------clare------

            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 = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public String readString() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = read();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        public boolean isSpaceChar(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

    }
}

