#include <bits/stdc++.h>
using namespace std;
void slv() {
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  map<char, int> cnt;
  for (auto it : s) cnt[it]++;
  for (auto it : cnt) {
    if (it.second > k) {
      cout << "NO\n";
      return;
    }
  }
  cout << "YES\n";
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int tc = 1;
  while (tc--) {
    slv();
  }
  return 0;
}
