#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  int mcnt = 0;
  for (int i = 0; i < n; i++) {
    int cnt = 0;
    for (int j = i + 1; j < n; j++) {
      if (s[i] == s[j]) cnt++;
    }
    mcnt = max(cnt, mcnt);
  }
  mcnt++;
  if (mcnt <= k) {
    cout << "YES";
  } else {
    cout << "NO";
  }
  return 0;
}
