#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
  int n, k;
  string s;
  cin >> n >> k >> s;
  map<char, int> map1;
  for (int i = 0; i < n; ++i) {
    map1[s[i]]++;
  }
  for (auto elem : map1) {
    if (elem.second > k) {
      puts("NO");
      return 0;
    }
  }
  puts("YES");
  return 0;
}
