#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
 
#define ALL(obj) (obj).begin(),(obj).end()
#define SPEED cin.tie(0);ios::sync_with_stdio(false);
 
template<class T> using PQ = priority_queue<T>;
template<class T> using PQR = priority_queue<T,vector<T>,greater<T>>;
 
constexpr long long MOD = (long long)1e9 + 7;
constexpr long long MOD2 = 998244353;
constexpr long long HIGHINF = (long long)1e18;
constexpr long long LOWINF = (long long)1e15;
constexpr long double PI = 3.1415926535897932384626433;
 
template <class T> vector<T> multivector(size_t N,T init){return vector<T>(N,init);}
template <class... T> auto multivector(size_t N,T... t){return vector<decltype(multivector(t...))>(N,multivector(t...));}
template <class T> void corner(bool flg, T hoge) {if (flg) {cout << hoge << endl; exit(0);}}
template <class T, class U>ostream &operator<<(ostream &o, const map<T, U>&obj) {o << "{"; for (auto &x : obj) o << " {" << x.first << " : " << x.second << "}" << ","; o << " }"; return o;}
template <class T>ostream &operator<<(ostream &o, const set<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const multiset<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const vector<T>&obj) {o << "{"; for (int i = 0; i < (int)obj.size(); ++i)o << (i > 0 ? ", " : "") << obj[i]; o << "}"; return o;}
template <class T, class U>ostream &operator<<(ostream &o, const pair<T, U>&obj) {o << "{" << obj.first << ", " << obj.second << "}"; return o;}
void print(void) {cout << endl;}
template <class Head> void print(Head&& head) {cout << head;print();}
template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {cout << head << " ";print(forward<Tail>(tail)...);}
template <class T> void chmax(T& a, const T b){a=max(a,b);}
template <class T> void chmin(T& a, const T b){a=min(a,b);}
void YN(bool flg) {cout << (flg ? "YES" : "NO") << endl;}
void Yn(bool flg) {cout << (flg ? "Yes" : "No") << endl;}
void yn(bool flg) {cout << (flg ? "yes" : "no") << endl;}


//O(N) N: vec.size();
template<class T> vector<T> SlideMost(vector<T>& vec, const int& width, function<int(T,T)> funcCompare){
    vector<T> res(vec.size());
    deque<T> deq;
    for(int i = 0; i < vec.size(); ++i) {
        while(deq.size() && funcCompare(vec[deq.back()],vec[i]) ) deq.pop_back();
        deq.push_back(i);
        res[i] = vec[deq.front()];
        if(i+1>=width && deq.front()==i+1-width) deq.pop_front();
    }
    return res;
}

	// auto Amax = SlideMost<int>(A,K,[&](ll l,ll r){return l <= r;});
	// auto Amin = SlideMost<int>(A,K,[&](ll l,ll r){return l >= r;});

//verify https://atcoder.jp/contests/cf16-tournament-round3-open/tasks/asaporo_d
//verify https://atcoder.jp/contests/agc038/tasks/agc038_b

int main() {
	ll N,K; cin >> N >> K;
	vector<double> A(N,0);
	for(int i = 0; i < N; ++i) {
		cin >> A[i];
		A[i] = (A[i]+1)/2.;
	}
	double sum = 0., ans = 0;
	for(int i = 0; i < K; ++i) {
		sum += A[i];
	}
	chmax(ans,sum);
	for(int i = K; i < N; ++i) {
		sum -= A[i-K];
		sum += A[i];
		chmax(ans,sum);
	}
	printf("%.10f",ans);




    return 0;
}


Measured memory_bytes: 1792