#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 17, lg = 17;
int tab[124];
int main() {
  ios::sync_with_stdio(0), cin.tie(0);
  int n, k;
  string s;
  cin >> n >> k >> s;
  for (int i = 0; i < s.size(); i++) {
    tab[s[i]]++;
  }
  bool ans = true;
  for (int i = 97; i < 123; i++) {
    if (tab[i] > k) ans = false;
  }
  if (ans)
    cout << "YES";
  else
    cout << "NO";
  return 0;
}
