#include <bits/stdc++.h>
using namespace std;
const int MAXN = 2e5 + 10;
const int inf = 1e9;
const int mod = 1e9 + 7;
long long a[26];
int32_t main() {
  long long n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  for (long long i = 0; i < n; i++) {
    long long t = s[i] - 'a';
    a[t]++;
  }
  long long ans = 0;
  for (long long i = 0; i < 26; i++) {
    if (a[i] > k) {
      cout << "NO" << endl;
      return 0;
    }
  }
  cout << "YES" << endl;
}
