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