#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  vector<int> ch(26);
  for (int i = 0; i < n; i++) {
    char c;
    cin >> c;
    ch[c - 'a']++;
  }
  if (k >= *max_element(ch.begin(), ch.end()))
    cout << "YES";
  else
    cout << "NO";
  return 0;
}
