```go
package main

import "fmt"

func main() {
	var n, k int
	fmt.Scan(&n, &k)
	var s string
	fmt.Scan(&s)
	counts := make([]int, 256)
	for i := 0; i < len(s); i++ {
		counts[s[i]]++
		if counts[s[i]] > k {
			fmt.Println("NO")
			return
		}
	}
	fmt.Println("YES")
}
```