#include<bits/stdc++.h>
using namespace std;
int main(){
	int i,n,a[21],c=0,max1=0,m;
	float max=0;
	cin>>n>>m;
	for(i=1;i<=n;i++){
		cin>>a[i];
	}
	for(i=1;i<=n-m+1;i++){
		c=0;
		for(int j=i;j<=i+m-1;j++){
			c+=a[j];
		}
		if(max<(c*1.00/m)){
			max=c*1.00/m;
		}
	}
	printf("%.2f",max);
	return 0;
}
/**************************************************************
	Problem: 1165
	User: houshanglin
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/