#include <bits/stdc++.h>
using namespace std;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
void io() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
}
int main() {
  io();
  int n, k, a[26] = {0}, f = 1;
  char t;
  cin >> n >> k;
  while (n--) {
    cin >> t;
    if (++a[t - 'a'] > k) {
      cout << "NO";
      f = 0;
      break;
    }
  }
  if (f == 1) cout << "YES";
}
