// Codeforces 1.12.6
// AtCoder 1.6
// Paiza 1.7.3
// TODO: Remove int64 funcs when `strconv.Atoi("123456345678")` returns the right result AT CODEFORCES.
package main

import (
	"bufio"
	"container/heap"
	"fmt"
	"os"
	"strconv"
	"strings"
)

// alias of fmt.Pirntln
func cout(a ...interface{}) {
	fmt.Println(a...)
}

func consoleIn() tokenizer {
	scn := bufio.NewScanner(os.Stdin)
	scn.Split(bufio.ScanWords)
	max := 1024 * 1024 * 32 // 32 Mib
	buf := make([]byte, max)
	scn.Buffer(buf, max)
	return tokenizer{Scanner: scn}
}

type tokenizer struct {
	Scanner *bufio.Scanner
}

func (in *tokenizer) tt() string {
	in.Scanner.Scan()
	return in.Scanner.Text()
}
func (in *tokenizer) ts(n int) []string {
	a := make([]string, n)
	for i := range a {
		a[i] = in.tt()
	}
	return a
}
func (in *tokenizer) ii() int {
	i, _ := strconv.Atoi(in.tt())
	return i
}
func (in *tokenizer) i2() (int, int) {
	return in.ii(), in.ii()
}
func (in *tokenizer) i3() (int, int, int) {
	return in.ii(), in.ii(), in.ii()
}
func (in *tokenizer) is(n int) []int {
	a := make([]int, n)
	for i := range a {
		a[i] = in.ii()
	}
	return a
}
func (in *tokenizer) im(h int, w int) [][]int {
	a := make([][]int, h)
	for y := range a {
		a[y] = make([]int, w)
		for x := range a[y] {
			a[y][x] = in.ii()
		}
	}
	return a
}

func (in *tokenizer) ib(base int) int {
	i, _ := strconv.ParseInt(in.tt(), base, 64)
	return int(i)
}

func (in *tokenizer) ff() float64 {
	i, _ := strconv.ParseFloat(in.tt(), 64)
	return i
}
func (in *tokenizer) fs(n int) []float64 {
	a := make([]float64, n)
	for i := range a {
		a[i] = in.ff()
	}
	return a
}

type intMath struct{}

func (*intMath) max(a ...int) int {
	t := a[0]
	for _, v := range a {
		if t < v {
			t = v
		}
	}
	return t
}
func (*intMath) min(a ...int) int {
	t := a[0]
	for _, v := range a {
		if t > v {
			t = v
		}
	}
	return t
}
func (*intMath) sum(a ...int) int {
	t := int(0)
	for _, v := range a {
		t += v
	}
	return t
}
func (*intMath) abs(a int) int {
	if a < 0 {
		return -a
	}
	return a
}
func (mt *intMath) gcd(a, b int) int {
	if a < b {
		a, b = b, a
	}
	if b == 0 {
		return a
	}
	return mt.gcd(b, a%b)
}
func (mt *intMath) lcm(a, b int) int {
	gcd := mt.gcd(a, b)
	return gcd * (a / gcd) * (b / gcd)
}

type stringify struct{}

// int slice to a line/lines.
func (*stringify) is(slice []int, vertical bool) string {
	ret := make([]string, len(slice))
	for i, x := range slice {
		ret[i] = strconv.FormatInt(int64(x), 10)
	}
	var separator string
	if vertical {
		separator = "\n"
	} else {
		separator = " "
	}
	return strings.Join(ret, separator)
}

// int matrix to lines.
func (stringify *stringify) im(matrix [][]int) string {
	ret := make([]string, len(matrix))
	for i := range matrix {
		ret[i] = stringify.is(matrix[i], false)
	}
	separator := "\n"
	return strings.Join(ret, separator)
}

// bool to Yes/No
func (*stringify) yesno(b bool) string {
	if b {
		return "Yes"
	}
	return "No"
}

// bool to Ok/Ng
func (*stringify) okng(b bool) string {
	if b {
		return "Ok"
	}
	return "Ng"
}

// comparable int slices
type cmpimat [][]int

func (p cmpimat) Len() int { return len(p) }
func (p cmpimat) Less(i, j int) bool {
	for x := range p[0] {
		if p[i][x] == p[j][x] {
			continue
		} else if p[i][x] < p[j][x] {
			return true
		} else {
			return false
		}
	}
	return false
}
func (p cmpimat) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
func (p *cmpimat) Push(slice interface{}) {
	x := slice.([]int)
	*p = append(*p, x)
	heap.Fix(p, len(*p)-1)
}
func (p *cmpimat) Pop() interface{} {
	// https://golang.org/pkg/container/heap/
	var ret []int
	n := len(*p)
	ret, *p = (*p)[n-1], (*p)[0:n-1]
	return ret
}
func (p *cmpimat) push(a ...int) {
	p.Push(a)
}
func (p *cmpimat) pop() []int {
	x := heap.Pop(p).([]int)
	return x
}

const imod int = 1000000007

var cin = consoleIn()
var mth = intMath{}
var str = stringify{}
var d4 = [][]int{{-1, 0}, {0, -1}, {1, 0}, {0, 1}}

func main() {
	n, k := cin.i2()
	p := cin.fs(n)

	ans := 0.0

	sum := 0.0
	for i, pp := range p {
		sum += pp
		if i < k-1 {
			continue
		} else if k <= i {
			sum -= p[i-k]
		}

		exp := (sum + float64(k)) / 2.0
		if ans < exp {
			ans = exp
		}
	}
	cout(ans)
}


Measured memory_bytes: 5120