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.Arrays;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Shurikat
 */
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);
        TaskB solver = new TaskB();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskB {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int n = in.nextInt();
            int k = in.nextInt();
            char[] input = in.nextLine(n).toCharArray();
            boolean[] used = new boolean[26];
            int[] first = new int[26];
            int[] last = new int[26];
            Arrays.fill(first, -1);
            Arrays.fill(last, -1);

            for (int i = 0; i < n; i++) {
                input[i] -= 'A';
            }
            for (int i = 0; i < n; i++) {
                used[input[i]] = true;
                if (first[input[i]] == -1) {
                    first[input[i]] = i;
                }
                last[input[i]] = i;
            }

            for (int i = 0; i < n; ++i) {
                int cnt = 0;
                for (int j = 0; j < 26; ++j) {
                    if (used[j]) {
                        if (i >= first[j] && i <= last[j]) {
                            ++cnt;
                        }
                    }
                }
                if (cnt > k) {
                    out.print("YES");
                    return;
                }
            }

            out.print("NO");
        }

    }

    static class InputReader implements AutoCloseable {
        private final InputStream in;
        private int capacity;
        private byte[] buffer;
        private int len = 0;
        private int cur = 0;

        public InputReader(InputStream stream) {
            this(stream, 100_000);
        }

        public InputReader(InputStream stream, int capacity) {
            this.in = stream;
            this.capacity = capacity;
            buffer = new byte[capacity];
        }

        private boolean update() {
            if (cur >= len) {
                try {
                    cur = 0;
                    len = in.read(buffer, 0, capacity);
                    if (len <= 0)
                        return false;
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
            }
            return true;
        }

        private int read() {
            if (update())
                return buffer[cur++];
            else return -1;
        }

        private boolean isSpace(int c) {
            return c == '\n' || c == '\t' || c == '\r' || c == ' ';
        }

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

        private int readSkipSpace() {
            int c;
            do {
                c = read();
            } while (isSpace(c));
            return c;
        }

        public int nextInt() {
            int c = readSkipSpace();
            if (c < 0)
                throw new InputMismatchException();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c == -1) break;
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res = res * 10 + c - '0';
                if (res < 0)
                    throw new InputMismatchException();
                c = read();
            } while (!isSpace(c));
            res *= sgn;
            return res;
        }

        public String nextLine(int initialCapacity) {
            StringBuilder res = new StringBuilder(initialCapacity);
            int c = readSkipSpace();
            do {
                res.append((char) (c));
                c = read();
            } while (!isEscape(c));
            return res.toString();
        }


        public void close() throws IOException {
            in.close();
        }

    }
}

