#include <bits/stdc++.h>
int main(int argc, const char* argv[]) {
  int n, k;
  std::cin >> n >> k;
  std::string s;
  std::cin >> s;
  std::vector<int> v(26);
  for (int i = 0; i < n; i++) v[s[i] - 'a']++;
  int count = 0, max = 0;
  for (int i = 0; i < 26; i++) {
    if (v[i] > 0) count++;
    max = std::max(max, v[i]);
  }
  if (max <= k)
    std::cout << "YES" << std::endl;
  else
    std::cout << "NO" << std::endl;
  return 0;
}
