#include <bits/stdc++.h>
using namespace std;
int main(){
	int i;
	double pp,mxp=-99999;
	cin>>i>>pp;
	double o[i];
	for(int p=0;p<i;p++)cin>>o[p];
	for(int p=0;p<=i-pp;p++){
		double sum=0;
		for(int q=0;q<pp;q++)sum+=o[p+q];
		if(mxp<sum/pp)mxp=sum/pp;
	}
	printf("%.2f",mxp);
	return 0;
}

/**************************************************************
	Problem: 1165
	User: zhouhongyi
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/