#include <bits/stdc++.h>
using namespace std;
int freq[100];
int main() {
  int n;
  cin >> n;
  int k;
  cin >> k;
  string s;
  cin >> s;
  for (int i = 0; i < n; i++) {
    freq[s[i] - 97]++;
  }
  int mx = 0;
  for (int i = 0; i < 26; i++) mx = max(mx, freq[i]);
  if (mx <= k)
    cout << "YES"
         << "\n";
  else
    cout << "NO"
         << "\n";
  return 0;
}
