package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
	"strings"
	// "regexp"
)

type Pair struct {
	p1, p2 interface{}
}

const (
	initialBufSize = 1e4
	maxBufSize     = 1e8
)

var (
	scanner = bufio.NewScanner(os.Stdin)
	writer  = bufio.NewWriter(os.Stdout)
)

func main() {
	buf := make([]byte, initialBufSize)
	scanner.Buffer(buf, maxBufSize)
	scanner.Split(bufio.ScanWords)

	N, K := readInt(), readInt()
	var P []int
	for i := 0; i < N; i++ {
		P = append(P, readInt()+1)
	}
	ans, s := 0.0, 0
	var q []int
	for i := 0; i < N; i++ {
		s += P[i]
		q = append(q, P[i])
		if len(q) > K {
			s -= q[0]
			q = q[1:]
		}
		if len(q) == K {
			ans = math.Max(float64(ans), float64(s))
		}
	}
	ans /= 2.0
	fmt.Printf("%.10f\n", ans)
}

/*==========================================
 *             Library
 *==========================================*/
func write(s string) {
	writer.WriteString(s)
}

func print() {
	writer.Flush()
}

// scanner.Split(bufio.ScanWords) をコメントアウトしないと使用不可
func readLine() (s string) {
	if scanner.Scan() {
		s = scanner.Text()
	}
	return s
}

func readInt() (read int) {
	scanner.Scan()
	read, err := strconv.Atoi(scanner.Text())
	if err != nil {
		panic(err)
	}
	return
}

func readFloat() (read float64) {
	scanner.Scan()
	read, err := strconv.ParseFloat(scanner.Text(), 64)
	if err != nil {
		panic(err)
	}
	return
}

func readRunes() (read []rune) {
	scanner.Scan()
	for _, v := range scanner.Text() {
		read = append(read, v)
	}
	return
}

func readString() (read string) {
	scanner.Scan()
	read = scanner.Text()
	return
}

func readStrings() (read []string) {
	scanner.Scan()
	for _, v := range scanner.Text() {
		read = append(read, string(v))
	}
	return
}

func s2i(s string) int {
	var intVal, e = strconv.Atoi(s)
	if e != nil {
		panic(e)
	}
	return intVal
}

func i2s(i int) string {
	var strVal = strconv.Itoa(i)
	return strVal
}

func s2f(s string) float64 {
	var floatVal, e = strconv.ParseFloat(s, 64)
	if e != nil {
		panic(e)
	}
	return floatVal
}

func sum(i []int) int {
	sum := 0
	for _, val := range i {
		sum += val
	}
	return sum
}

func split(s string) []string {
	return strings.Fields(s)
}

func strAry2intAry(strs []string) []int {
	var ret = make([]int, 0, len(strs))
	for _, str := range strs {
		var intVal, e = strconv.Atoi(string(str))
		if e != nil {
			panic(e)
		}
		ret = append(ret, intVal)
	}
	return ret
}

func intAry2strAry(nums []int) []string {
	var ret = make([]string, 0, len(nums))
	for _, num := range nums {
		var strVal = strconv.Itoa(num)
		ret = append(ret, strVal)
	}
	return ret
}

func ary2str(strs []string) string {
	return strings.Join(strs, " ")
}

func reverse(res []int) []int {
	for i, j := 0, len(res)-1; i < j; i, j = i+1, j-1 {
		res[i], res[j] = res[j], res[i]
	}
	return res
}

func initalize(res []int, init int) {
	if len(res) == 0 {
		return
	}
	res[0] = init
	for i := 0; i < len(res); i++ {
		copy(res[i:], res[:i])
	}
}

//
// func regexpExample() {
//     // Your code here!
//     var str = "13:20"
//     r := regexp.MustCompile(`(\d+):(\d+)`)
//     fmt.Println(r.FindStringSubmatch(str))
// }

// type Country struct {
//     gold int
//     silver int
//     blonze int
// }

// // 複数ソートのサンプル
// func stableSortExample() []Country{
//     var country = []Country {
//         {gold:1, silver:2, blonze:3},
//         {gold:1, silver:2, blonze:3},
//         {gold:1, silver:3, blonze:2},
//         {gold:1, silver:3, blonze:3},
//     }
//     sort.SliceStable(country, func(i, j int) bool { return country[i].blonze > country[j].blonze })
//     sort.SliceStable(country, func(i, j int) bool { return country[i].silver > country[j].silver })
//     sort.SliceStable(country, func(i, j int) bool { return country[i].gold > country[j].gold })
//     return country
// }


Measured memory_bytes: 9984