#include <bits/stdc++.h>
using namespace std;
int test[26];
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  int n, m, t(0), cnt(0);
  string s, now = "";
  cin >> n >> m >> s;
  t = s.size();
  for (int i = 0; i < t; i++) {
    test[s[i] - 'a']++;
  }
  for (int i = 0; i < 26; i++) {
    if (test[i] > m) {
      return cout << "NO\n", 0;
    }
  }
  cout << "YES\n";
  return 0;
}
