#include <bits/stdc++.h>
using namespace std;
int main() {
  char a[1000];
  int b[1000] = {0};
  int i, j, n, k, t, m = 0, mark = 1;
  scanf("%d%d", &n, &k);
  scanf("%s", a);
  sort(a, a + n);
  for (i = 0, j = i + 1; j < n; j++, i++) {
    if (a[i] == a[j])
      b[m]++;
    else
      m++;
  }
  mark = b[0];
  for (i = 0; i < n; i++) {
    if (mark < b[i]) mark = b[i];
  }
  mark = mark + 1;
  if (n <= k)
    printf("YES");
  else {
    if (mark > k)
      printf("NO");
    else
      printf("YES");
  }
}
