#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  char ch[n + 1];
  cin >> ch;
  int ar[200] = {0};
  for (int i = 0; i < strlen(ch); i++) {
    ar[ch[i]]++;
  }
  sort(ar, ar + 200);
  if (ar[199] > k)
    cout << "NO" << endl;
  else
    cout << "YES" << endl;
  return 0;
}
