#include <bits/stdc++.h>
using namespace std;
int main(){
	int n,i,m,a[110]={0},j,h=0;
	float z=0;
	cin>>n>>m;
	for(i=0;i<n;i++){
		cin>>a[i];
	} 
	for(i=0;i<n-m+1;i++){
		h=0;
		for(j=i;j<=i+2;j++){
			h=h+a[j];
		}
		if(z<h){
			z=h;
		}
	}
	z=z/m;
	printf("%.2f",z);
	
}


/**************************************************************
	Problem: 1165
	User: 15159808820
	Language: C++
	Result: Accepted
	Time:7 ms
	Memory:2072 kb
****************************************************************/