#include <bits/stdc++.h>
int a[26];
int main() {
  int m, n;
  char str[111];
  scanf("%d%d", &m, &n);
  scanf("%s", str);
  for (int j = 0; j < m; j++) a[str[j] - 'a']++;
  for (int j = 0; j < 26; j++)
    if (a[j] > n) return 0 * printf("NO");
  printf("YES");
  return 0;
}
