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