#include <bits/stdc++.h>
using namespace std;
int mp[30];
int main() {
  int n, k;
  scanf("%d %d", &n, &k);
  int ans = 0;
  for (int i = 1; i <= n; i++) {
    char now;
    scanf(" %c", &now);
    mp[now - 'a' + 1]++;
    ans = max(ans, mp[now - 'a' + 1]);
  }
  if (ans > k)
    printf("NO\n");
  else
    printf("YES\n");
  return 0;
}
