#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  int ctr[26] = {0};
  int i;
  for (i = 0; i < s.length(); i++) {
    ctr[s[i] - 'a']++;
    if (ctr[s[i] - 'a'] > k) {
      cout << "NO";
      return 0;
    }
  }
  cout << "YES";
  return 0;
}
