#include <bits/stdc++.h>
int cnt[30];
int main() {
  int n, k;
  char ch[110];
  scanf("%d%d%s", &n, &k, ch + 1);
  for (int i = 1; i <= n; ++i)
    if (++cnt[ch[i] - 'a' + 1] > k) {
      std::cout << "NO" << std::endl;
      return 0;
    }
  std::cout << "YES" << std::endl;
  return 0;
}
