#include <bits/stdc++.h>
using namespace std;
int main(int argc, char **argv) {
  int n, k;
  char s[101];
  scanf("%d%d%s", &n, &k, s);
  int freq[26] = {}, mx = 0;
  for (int i = 0; i < n; ++i) mx = max(mx, ++freq[s[i] - 'a']);
  if (mx <= k)
    puts("YES");
  else
    puts("NO");
  return 0;
}
