#include <bits/stdc++.h>
using namespace std;
char mp[1000];
int main() {
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  for (int i = 0; i < n; i++) {
    mp[s[i] - 'a']++;
    if (mp[s[i] - 'a'] > k) {
      cout << "NO";
      return 0;
    }
  }
  cout << "YES";
}
