#include <bits/stdc++.h>
using namespace std;
char s[999];
int ans[28];
int n, k;
int main() {
  cin >> n >> k;
  for (int i = 1; i <= n; i++) {
    cin >> s[i];
    ans[s[i] - 'a']++;
    if (ans[s[i] - 'a'] > k) {
      cout << "NO" << endl;
      return 0;
    }
  }
  cout << "YES" << endl;
  return 0;
}
