#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, count;
  char s[150], *p;
  cin >> n >> k >> s;
  for (int i = 'a'; i <= 'z'; i++) {
    count = 0;
    for (p = s; *p; p++)
      if (*p == i) count++;
    if (count > k) {
      cout << "NO" << endl;
      return 0;
    }
  }
  cout << "YES" << endl;
}
