#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  int n, k;
  cin >> n >> k;
  string s;
  int arr[127];
  fill(arr, arr + 127, 0);
  cin >> s;
  for (int i = 0; i < s.length(); ++i) {
    arr[s[i]]++;
  }
  for (int i = 0; i < 127; ++i) {
    if (arr[i] > k) {
      cout << "NO\n";
      return 0;
    }
  }
  cout << "YES\n";
  return 0;
}
