#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  int n, k;
  char d[101];
  int cnt[26];
  memset(cnt, 0, sizeof(cnt));
  int i;
  cin >> n >> k;
  cin >> d;
  int max = 0;
  for (i = 0; i < n; i++) {
    cnt[d[i] - 'a']++;
    if (cnt[d[i] - 'a'] > max) {
      max = cnt[d[i] - 'a'];
    }
  }
  if (max > k) {
    cout << "NO" << endl;
  } else {
    cout << "YES" << endl;
  }
  return 0;
}
