/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author
 */

#include <iostream>
#include <fstream>

// https://gist.githubusercontent.com/shibh308/f7ab83aa58b95322f0c5a353121bdecb/raw/b19dcf74b08f80f601d7a5792f2578b6ff9155bf/template.cpp
#include <bits/stdc++.h>
typedef long long ll;
const int INF = 1e9;
const int MOD = 1e9+7;
const ll LINF = 1e18;
using namespace std;

#define dump(x)  cout << #x << " = " << (x) << endl;
#define YES(n) cout << ((n) ? "YES" : "NO"  ) << endl
#define Yes(n) cout << ((n) ? "Yes" : "No"  ) << endl
#define POSSIBLE(n) cout << ((n) ? "POSSIBLE" : "IMPOSSIBLE"  ) << endl
#define Possible(n) cout << ((n) ? "Possible" : "Impossible"  ) << endl

#define SANKOU(n,a,b) cout << ((n) ? (#a) : (#b) ) << endl

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) for(int i=0;i<(n);++i)
#define REPR(i,n) for(int i=n;i>=0;i--)

#define FOREACH(x,a) for(auto& (x) : (a) )

#define SCOUT(x) cout<<(x)<<" "
#define ENDL cout<<endl

#define VECCIN(x) for(auto&youso_: (x) )cin>>youso_
#define VECCOUT(x) for(auto&youso_: (x) )cout<<youso_<<" ";cout<<endl

#define ALL(obj) (obj).begin(),(obj).end()

#define COUT(x) cout<<(x)<<endl

void CINT(){}
template <class... Tail>
void CINT(istream& cins, Tail&&... tail){
    return;
}
template <class Head,class... Tail>
void CINT(istream& cin, Head&& head,Tail&&... tail){
    cin>>head;
    CINT(cin, move(tail)...);
}

void MCINT(){}
template <class... Tail>
void MCINT(istream& cins, Tail&&... tail){
    return;
}
template <class Head,class... Tail>
void MCINT(istream& cin, Head&& head,Tail&&... tail){
    cin>>head.val;
    MCINT(cin, move(tail)...);
}

#define CIN(...) int __VA_ARGS__;CINT(cin,__VA_ARGS__)
#define LCIN(...) ll __VA_ARGS__;CINT(cin,__VA_ARGS__)
#define SCIN(...) string __VA_ARGS__;CINT(cin,__VA_ARGS__)
#define MCIN(...) mint __VA_ARGS__;MCINT(cin,__VA_ARGS__)

template <class T = ll>
T IN(){T x;cin>>x;return (x);}

template <class Head>
void VT(Head head){}
template <class Head,class Seco,class... Tail>
void VT(Head&& head,Seco&& seco,Tail&&... tail){
    seco.resize(head);
    VT(head,move(tail)...);
}
void VT2(){}
template <class Head,class... Tail>
void VT2(Head&& head,Tail&&... tail){
    VECCIN(head);
    VT2(move(tail)...);
}

template <class Head>
void VT3(Head&& head){}
template <class Head,class Seco,class... Tail>
void VT3(Head&& head,Seco&& seco,Tail&&... tail){
    seco[head]=IN();
    VT3(head,move(tail)...);
}

#define VCEACH(n,...) V __VA_ARGS__;VT(n,__VA_ARGS__);VT2(__VA_ARGS__);
#define VCREP(n,...) V __VA_ARGS__;VT(n,__VA_ARGS__);REP(i,n)VT3(i,__VA_ARGS__);

#define P pair<int,int>
#define V vector<int>
#define VV vector<V>
#define M map<int,int>
#define S set<int>
#define PQ priority_queue<int>
#define PQG priority_queue<int,V,greater<int>>

#define pb(a) push_back(a)
#define mp make_pair

// https://qiita.com/drken/items/3b4fdf0a78e7a138cd9a
template<int MOD> struct Fp {
    long long val;
    constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
        if (val < 0) val += MOD;
    }
    constexpr int getmod() { return MOD; }
    constexpr Fp operator - () const noexcept {
        return val ? MOD - val : 0;
    }
    constexpr Fp operator + (const Fp& r) const noexcept { return Fp(*this) += r; }
    constexpr Fp operator - (const Fp& r) const noexcept { return Fp(*this) -= r; }
    constexpr Fp operator * (const Fp& r) const noexcept { return Fp(*this) *= r; }
    constexpr Fp operator / (const Fp& r) const noexcept { return Fp(*this) /= r; }
    constexpr Fp& operator += (const Fp& r) noexcept {
        val += r.val;
        if (val >= MOD) val -= MOD;
        return *this;
    }
    constexpr Fp& operator -= (const Fp& r) noexcept {
        val -= r.val;
        if (val < 0) val += MOD;
        return *this;
    }
    constexpr Fp& operator *= (const Fp& r) noexcept {
        val = val * r.val % MOD;
        return *this;
    }
    constexpr Fp& operator /= (const Fp& r) noexcept {
        long long a = r.val, b = MOD, u = 1, v = 0;
        while (b) {
            long long t = a / b;
            a -= t * b; swap(a, b);
            u -= t * v; swap(u, v);
        }
        val = val * u % MOD;
        if (val < 0) val += MOD;
        return *this;
    }
    constexpr bool operator == (const Fp& r) const noexcept {
        return this->val == r.val;
    }
    constexpr bool operator != (const Fp& r) const noexcept {
        return this->val != r.val;
    }
    friend constexpr ostream& operator << (ostream &os, const Fp<MOD>& x) noexcept {
        return os << x.val;
    }
    friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
        if (n == 0) return 1;
        auto t = modpow(a, n / 2);
        t = t * t;
        if (n & 1) t = t * a;
        return t;
    }
};

using mint = Fp<MOD>;


class DDiceInLine {
public:
    void solve(std::istream& cin, std::ostream& cout) {
        CIN(n,k);
        vector<float> p(n);
        VECCIN(p);

        REP(i,p.size()){
            p[i] = (p[i] + 1)/2.0;
        }

        double m = -1;
        double sum = 0;
        REP(i,k){
            sum+=p[i];
        }

        m=max(m,sum);

        FOR(i, 0, p.size() - k){
            sum-=p[i];
            sum+=p[i+k];
            m=max(m,sum);
        }
        cout << fixed;
        cout << setprecision(12) << m << endl;
    }
};

int main() {
	DDiceInLine solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	solver.solve(in, out);
	return 0;
}


Measured memory_bytes: 1024