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

#include <bits/stdc++.h>
using namespace std;
long long gcd(long long a, long long b) {
  if ((a == 0) || (b == 0)) return a + b;
  return gcd(b, a % b);
}
long long pow_mod(long long a, long long b) {
  long long res = 1;
  while (b) {
    if (b & 1) res = (res * a) % 1000000007;
    a = (a * a) % 1000000007;
    b >>= 1;
  }
  return (res % 1000000007);
}
long long pow_2(long long a, long long b) {
  long long res = 1;
  while (b) {
    if (b & 1) res = (res * a);
    a = (a * a);
    b >>= 1;
  }
  return res;
}
bool isPrime(long long a) {
  for (long long i = 3; (i * i) <= a; i += 2) {
    if ((a % i) == 0) return false;
  }
  if ((a != 2) && ((a % 2) == 0)) return false;
  if (a == 1) return false;
  return true;
}
string con_ll_to_str(long long a) {
  stringstream mystr;
  mystr << a;
  return mystr.str();
}
long long con_str_to_ll(string st) {
  long long numb = 0;
  long long len = st.size(), i, j = 0;
  for (i = len - 1; i >= 0; i--) {
    numb += (pow_2(10, j) * (st[i] - '0'));
    j++;
  }
  return numb;
}
long long baseno_to_decimal(string st, long long basee) {
  long long i = 0, j, anss = 0;
  for (j = (int)st.length() - 1; j >= 0; j--) {
    anss += (st[j] - '0') * pow_2(basee, i);
    i++;
  }
  return anss;
}
string decimal_to_string(long long num, long long basee) {
  long long i = 0, j, opop;
  string anss = "";
  vector<string> stri;
  stri.push_back("0");
  stri.push_back("1");
  stri.push_back("2");
  stri.push_back("3");
  stri.push_back("4");
  stri.push_back("5");
  stri.push_back("6");
  stri.push_back("7");
  stri.push_back("8");
  stri.push_back("9");
  stri.push_back("A");
  stri.push_back("B");
  stri.push_back("C");
  stri.push_back("D");
  stri.push_back("E");
  stri.push_back("F");
  if (num == 0) {
    return "0";
  }
  while (num) {
    opop = num % basee;
    anss += stri[opop];
    num /= basee;
  }
  reverse(anss.begin(), anss.end());
  return anss;
}
const int dx[] = {0, 1, 1, 1, 0, -1, -1, -1};
const int dy[] = {1, 1, 0, -1, -1, -1, 0, 1};
long long arr[30];
set<long long> se;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  long long T, N, K, i, j;
  string st;
  cin >> N >> K;
  cin >> st;
  for (i = 0; i <= (int)st.length() - 1; i++) {
    arr[st[i] - 'A']++;
  }
  for (i = 0; i <= (int)st.length() - 1; i++) {
    se.insert(st[i] - 'A');
    if (se.size() > K) {
      cout << "YES";
      return 0;
    }
    arr[st[i] - 'A']--;
    if (!arr[st[i] - 'A']) {
      se.erase(st[i] - 'A');
    }
  }
  cout << "NO";
  return 0;
}
