#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  unordered_map<int, int> ump;
  for (int i = 0; i < s.size(); i++) {
    ump[s[i] - 'a']++;
  }
  for (auto uj : ump) {
    if (uj.second > k) {
      cout << "NO" << endl;
      exit(0);
    }
  }
  cout << "YES" << endl;
}
