#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  string l;
  cin >> l;
  sort(l.begin(), l.end());
  int a = 0, ans = 0;
  char p = l[0];
  for (;;) {
    if (l[0] == p) {
      a++;
      l.erase(l.begin());
    } else {
      ans = max(ans, a);
      if (l.size() > 0) {
        p = l[0];
        a = 0;
      } else
        break;
    }
  }
  if (ans > k)
    cout << "NO";
  else
    cout << "YES";
  return 0;
}
