#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  string s;
  cin >> n >> k;
  cin >> s;
  int isThere[26];
  for (int i = 0; i < 26; i++) isThere[i] = 0;
  bool ok = true;
  for (int i = 0; i < n; i++) {
    isThere[s[i] - 'a']++;
    if (isThere[s[i] - 'a'] > k) {
      ok = false;
      break;
    }
  }
  if (ok)
    cout << "YES";
  else
    cout << "NO";
}
