#include <bits/stdc++.h>
using namespace std;
int main() {
  int a[30];
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  memset(a, 0, sizeof(a));
  for (int i = 0; i < s.size(); i++) {
    int p = s[i] - 96;
    a[p]++;
  }
  for (int i = 1; i <= 26; i++) {
    if (a[i] > k) {
      cout << "NO" << endl;
      return 0;
    }
  }
  cout << "YES" << endl;
  return 0;
}
