```go
package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {
	in := bufio.NewReader(os.Stdin)

	var n, k int
	fmt.Fscan(in, &n, &k)

	var s string
	fmt.Fscan(in, &s)

	cnt := make([]int, 26)
	for _, ch := range s {
		if ch >= 'a' && ch <= 'z' {
			cnt[ch-'a']++
		}
	}

	for i := 0; i < 26; i++ {
		if cnt[i] > k {
			fmt.Print("NO")
			return
		}
	}
	fmt.Print("YES")
}
```