#include <bits/stdc++.h>
using namespace std;
#define lli long long int
#define mp(a,b) make_pair(a,b)
#define pb(a) push_back(a)
#define fr(i,n) for(i=0;i<n;i++)
#define F first
#define S second
#define fast std::ios_base::sync_with_stdio(false)
#define mod 1000000007
#define pi 3.14159265
int main() {
	fast;
	lli n,k;
	cin>>n>>k;
	lli a[n],i;
	fr(i,n)
	{
		cin>>a[i];
	}
	double b[n],sum=0,ans=0;
	fr(i,n)
	{
		b[i]=(float)(a[i]+1)/2;
	}
	fr(i,k)
	{
		sum+=b[i];
	}
	ans=sum;
	for(i=k;i<n;i++)
	{
		sum-=b[i-k];
		sum+=b[i];
		ans=max(ans,sum);
	}
	cout<<fixed<<setprecision(8)<<ans;
	
	return 0;
}