#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define INF 1LL<<62
#define inf 1000000007

int main() {
	long long n,k;
	cin>>n>>k;
	long double ch[200010];
	for(ll i=0;i<n;i++){
		cin>>ch[i];
	}
	long double ans=0;
	for(ll i=0;i<k;i++){
		long double now=ch[i];
		now=(ch[i]+1)/2;
		ans+=now;
	}
	//printf("%.10Lf\n",ans);	
	long double cnt=ans;
	for(ll i=0;i+k<n;i++){
		cnt+=-(ch[i]+1)/2+(ch[i+k]+1)/2;
		ans=max(ans,cnt);
	}
	printf("%.10Lf",ans);
	// your code goes here
	return 0;
}
#include<iostream>
#include<vector>
#include<map>
#include<set>
#include<cassert>
#include<cassert>
#include<unordered_map>
#include<unordered_set>
#include<functional>
#include<queue>
#include<stack>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<sstream>
#include<iomanip>
#include<cstdio>
#include<cstdlib>
#include<numeric>
#include<random>
#include<chrono>
#include<bitset>
using namespace std;

#define all(x) (x).begin(), (x).end()
#define pb push_back
#define xx first
#define yy second
#define sz(x) (int)(x).size()
#define gc getchar
#define IO ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0)
#define mp make_pair

#ifndef ONLINE_JUDGE
#  define LOG(x) (cerr << #x << " = " << (x) << endl)
#else
#  define LOG(x) ((void)0)
#endif

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;

const double PI=acos(-1);
const ll INF = 1LL<<62;
const ll MINF = -(1LL<<62);

template<typename T> T getint() {
	T val=0;
	char c;
	
	bool neg=false;
	while((c=gc()) && !(c>='0' && c<='9')) {
		neg|=c=='-';
	}

	do {
		val=(val*10)+c-'0';
	} while((c=gc()) && (c>='0' && c<='9'));

	return val*(neg?-1:1);
}

//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count()); uniform_int_distribution<int>(0, n-1)(rng)


int main() {
	IO;
	ll a,b;
	cin>>a>>b;
	cout<<a*b<<"\n";
	return 0;
}
