#include <bits/stdc++.h>
using namespace std;
string s;
map<char, int> mp;
int main() {
  int n, k;
  while (cin >> n >> k) {
    cin >> s;
    mp.clear();
    int yes = 1;
    for (int i = 0; i < n; i++) {
      mp[s[i]]++;
      if (mp[s[i]] > k) yes = 0;
    }
    if (yes)
      puts("YES");
    else
      puts("NO");
  }
  return 0;
}
