#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  int i, a[27] = {0};
  for (i = 0; i < s.length(); i++) {
    a[s[i] - 'a']++;
  }
  int max = a[0], j = 0;
  for (i = 0; i < 27; i++) {
    if (a[i] > max) {
      max = a[i];
      j = i;
    }
  }
  if (max > k)
    cout << "NO";
  else
    cout << "YES";
  return 0;
}
