#include <bits/stdc++.h>
using namespace std;
int num[30] = {0};
int n, k;
char s[10005];
int main() {
  scanf("%d%d%s", &n, &k, s);
  for (int i = 0; i < n; i++) num[s[i] - 'a']++;
  bool ok = true;
  for (int i = 0; i < 26; i++)
    if (num[i] > k) ok = false;
  if (ok)
    printf("YES\n");
  else
    printf("NO\n");
  return 0;
}
