#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  int n, k;
  cin >> n >> k;
  vector<int> cnt(26, 0);
  string s;
  cin >> s;
  bool ans = 1;
  for (int i = 0; i < n; ++i) {
    cnt[s[i] - 'a']++;
    if (cnt[s[i] - 'a'] > k) ans = 0;
  }
  cout << (ans ? "YES" : "NO") << endl;
  return 0;
}
