#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <cctype>
#include <climits>
#include <string>
#include <bitset>
#include <cfloat>
#include <unordered_set>
#pragma GCC optimize("Ofast")
using namespace std;
typedef long double ld;
typedef long long int ll;
typedef unsigned long long int ull;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<ll> vll;
typedef vector<pair<int,int> > vpii;
typedef vector<vector<int> > vvi;
typedef vector<vector<char> > vvc;
typedef vector<vector<string> > vvs;
typedef vector<vector<ll> > vvll;
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define irep(it, stl) for(auto it = stl.begin(); it != stl.end(); it++)
#define drep(i,n) for(int i = (n) - 1; i >= 0; --i)
#define fin(ans) cout << (ans) << '\n'
#define STI(s) atoi(s.c_str())
#define mp(p,q) make_pair(p, q)
#define pb(n) push_back(n)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define Sort(a) sort(a.begin(), a.end())
#define Rort(a) sort(a.rbegin(), a.rend())
#define MATHPI acos(-1)
#define itn int;
int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
template <class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;} return 0;}
template <class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;} return 0;}
struct io{io(){ios::sync_with_stdio(false);cin.tie(0);}};
const int INF = INT_MAX;
const ll LLINF = 1LL<<60;
const ll MOD = 1000000007;
const double EPS = 1e-9;

class Monoid {
public:
    // 単位元
    double unit;
    
    Monoid() {
        // 単位元
        unit = 0;
    }

    // 演算関数
    double calc(double a, double b) {
        return a+b; 
    }
};

class SegmentTree {
public:
    // セグメント木の葉の要素数
    int n;

    // セグメント木
    vector<double> tree;

    // モノイド
    Monoid mono;

    SegmentTree(vector<double> &v) {
        n = 1 << (int)ceil(log2(v.size()));
        tree = vector<double>(n << 1);
        for(int i = 0; i < v.size(); i++) {
            update(i, v[i]);
        }
        for(int i = v.size(); i < n; i++) {
            update(i, mono.unit);
        }
    }

    // k番目の値(0-indexed)をxに変更
    void update(int k, double x) {
        k += n;
        tree[k] = x;
        for(k = k >> 1; k > 0; k >>= 1){
            tree[k] = mono.calc(tree[k << 1 | 0], tree[k << 1 | 1]);
        }
    }

    // [l, r)の最小値(0-indexed)を求める．
    double query(int l, int r) {
        double res = mono.unit;
        l += n;
        r += n;
        while(l < r) {
            if(l & 1) {
                res = mono.calc(res, tree[l++]);
            }
            if(r & 1) {
                res = mono.calc(res, tree[--r]);
            }
            l >>= 1;
            r >>= 1;
        }
        return res;
    }
    int operator[](int k) {
        // st[i]で添字iの要素の値を返す
        if(k - n >= 0 || k < 0) {
            return -INF;
        }
        return tree[tree.size()-n+k];
    }

    void show() {
        int ret = 2;
        for(int i = 1; i < 2*n; i++) {
            cout << tree[i] << " ";
            if(i == ret - 1) {
                cout << endl;
                ret <<= 1;
            }
        }
        cout << endl;
    }
};

int main(void) {
    
  /*  
    int n;
    cin>>n;
    vi a(n);
    rep(i,n)cin>>a[i];
    set<int> s;
    rep(i,n)s.insert(a[i]);
    if(s.size()!=n)fin("NO");
    else fin("YES");
*/
    int n,k;
    cin>>n>>k;
    vd v(n);
    rep(i,n){
        cin>>v[i];
        v[i] = (v[i]-1.0)/2.0+1.0;
    }
    SegmentTree st(v);
    double ans=0.0;
    for(int i=0;i<=n-k;i++){
        // cout<<st.query(i,i+k)<<endl;
        chmax(ans, st.query(i,i+k));
    }
    printf("%.6f\n",ans);
}


Measured memory_bytes: 5888