```go
package main

import "fmt"

func main() {
	var n, k int
	fmt.Scan(&n, &k)
	a := make([]int64, n)
	for i := 0; i < n; i++ {
		fmt.Scan(&a[i])
	}
	prefix := make([]int64, n+1)
	for i := 1; i <= n; i++ {
		prefix[i] = prefix[i-1] + a[i-1]
	}
	var maxAvg float64 = 0
	for i := 0; i < n; i++ {
		for j := i+k-1; j < n; j++ {
			sumv := prefix[j+1] - prefix[i]
			lenv := j - i + 1
			avg := float64(sumv) / float64(lenv)
			if avg > maxAvg {
				maxAvg = avg
			}
		}
	}
	fmt.Printf("%.10f\n", maxAvg)
}
```