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

#!/usr/bin/env python
from __future__ import division, print_function

import os
import sys
from io import BytesIO, IOBase

if sys.version_info[0] < 3:
    from __builtin__ import xrange as range
    from future_builtins import ascii, filter, hex, map, oct, zip


class FastI(BytesIO):
    newlines = 0

    def __init__(self, fd=0, bufsize=8192):
        self.fd = fd
        self.bufsize = bufsize
        self.read = lambda: super(FastI, self).read() if self.newlines else os.read(self.fd, os.fstat(self.fd).st_size)

    def readline(self):
        while self.newlines == 0:
            b, ptr = os.read(self.fd, max(os.fstat(self.fd).st_size, self.bufsize)), self.tell()
            self.seek(0, 2), self.write(b), self.seek(ptr)
            self.newlines += b.count(b'\n') + (not b)

        self.newlines -= 1
        return super(FastI, self).readline()


class FastO(IOBase):
    def __init__(self, fd=1):
        stream = BytesIO()
        self.flush = lambda: os.write(fd, stream.getvalue()) and not stream.truncate(0) and stream.seek(0)
        self.write = stream.write if sys.version_info[0] < 3 else lambda b: stream.write(b.encode())


class ostream:
    def __lshift__(self, a):
        if a == endl:
            sys.stdout.write('\n')
            sys.stdout.flush()
        else:
            sys.stdout.write(str(a))
        return self


sys.stdin, sys.stdout = FastI(), FastO()
cout, endl = ostream(), object()

input = sys.stdin.readline


def main():
    n, k = map(int, input().split())
    s = [x - 65 for x in input().rstrip(b'\r\n')]

    o, c = [-1] * 26, [-1] * 26
    for i in range(n):
        if o[s[i]] == -1:
            o[s[i]] = i
        c[s[i]] = i

    j = 0
    for i in range(n):
        if o[s[i]] == i:
            j += 1
        if j > k:
            print('YES')
            return
        if c[s[i]] == i:
            j -= 1

    print('NO')


if __name__ == '__main__':
    main()
