#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  int n, k, a[26], flag = 0;
  string s;
  cin >> n >> k;
  cin >> s;
  memset(a, 0, sizeof(a));
  for (int i = 0; i < n; i++) {
    a[s[i] - 'a']++;
  }
  if (*max_element(a, a + 26) > k)
    cout << "NO";
  else
    cout << "YES";
  return 0;
}
