#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  map<char, int> mpr;
  char alpha;
  for (int i = 0; i < n; i++) {
    cin >> alpha;
    mpr[alpha]++;
    if (mpr[alpha] > k) {
      cout << "No";
      exit(0);
    }
  }
  cout << "Yes";
  return 0;
}
