#include <bits/stdc++.h>
using namespace std;
const int maxn = 100 + 10;
char base[] = "doge";
char s[maxn];
int num[maxn];
int gcd() {}
int main() {
  int n, k;
  scanf("%d%d", &n, &k);
  if (n) scanf("%s", s);
  memset(num, 0, sizeof num);
  for (int i = 0; i < n; i++) num[s[i] - 'a']++;
  int ok = 1;
  for (int i = 0; i < 26; i++) {
    if (num[i] > k) ok = 0;
  }
  if (ok)
    printf("YES\n");
  else
    printf("NO\n");
  return 0;
}
