#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const double pi = acos(-1);
#define FOR(i,a,b) for (ll i=(a),__last_##i=(b);i<__last_##i;i++)
#define RFOR(i,a,b) for (ll i=(b)-1,__last_##i=(a);i>=__last_##i;i--)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) RFOR(i,0,n)
#define __GET_MACRO3(_1, _2, _3, NAME, ...) NAME
#define rep(...) __GET_MACRO3(__VA_ARGS__, FOR, REP)(__VA_ARGS__)
#define rrep(...) __GET_MACRO3(__VA_ARGS__, RFOR, RREP)(__VA_ARGS__)
#define vi vector<int>
template<typename T> istream &operator>>(istream &is, vector<T> &v){ for (auto &x : v) is >> x; return is; }
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v) {
    REP(i,v.size()){if(i)os<<" ";os<<v[i];}return os;}
template<typename T> ostream& operator<<(ostream& os, const vector<vector<T>>& v) {
    REP(i,v.size()){if(i)os<<endl;os<<v[i];}return os;}

const int MOD = 1e9 + 7;
const int INF = 1 << 30;


class UnionFind{
private:
    vector<int> par,rank,sizes;
    int tree;

public:
    explicit UnionFind(const int n) : tree(n){
        par.resize(n+1);
        rank.resize(n+1);
        sizes.resize(n+1);
        rep(i,n){
            par[i] = i;
            sizes[i] = 1;
        }
    }

    ~UnionFind() = default;

    int root(int x){
        if(par[x] == x) return x;
        else return par[x] = root(par[x]);
    }

    int find(int x){ return root(x);}

    void unite(int x, int y) {
        x = root(x);
        y = root(y);
        if (x == y) return;

        if (rank[x] < rank[y]) swap(x, y);
        par[y] = x;
        tree--;
        sizes[x] += sizes[y];
        if (rank[x] == rank[y]) rank[x]++;
    }

    bool same(int x, int y){
        return root(x) == root(y);
    }

    int size(int x){
        return sizes[root(x)];
    }

    int sumTree(){
        return tree;
    }
};

template <typename T>
class segmentTree{
private:
    int n;
    vector<T> arr;
public:

    explicit segmentTree(vector<T> v){
        ll sz = v.size();
        //while()

        arr.resize(2*n - 1);

    }

    T &operator[](int n){ return arr[n];}


};

template <int mod>
class ModInt{
private:
    ll _num;

public:
    ModInt(){_num = 0;}
    explicit ModInt(ll value){_num = value%mod;}

    ModInt operator+(ModInt &x){ return ModInt(_num + x._num);}
    ModInt operator-(ModInt &x){ return ModInt(_num - x._num);}
    ModInt operator*(ModInt &x){ return ModInt(_num * x._num);}
    friend ostream &operator<<(ostream &os, const ModInt &x){os<<x._num; return os;}
    friend istream &operator>>(istream &is, ModInt &x){is>>x._num; return is;}
};
using mint = ModInt<MOD>;

ll power(ll m, ll n, ll mod){
    ll res = 1;
    while(n > 0){
        if(n & 1) res = ((res%mod) * (m%mod))%mod;
        m = (m * m)%mod;
        n >>= 1;
    }
    return res;
}
//const ll MOD=998244353ll;


using Matrix = vector<vector<ll>>;

Matrix multi(Matrix a,Matrix b){
    ll n = a.size();
    Matrix c(n,vector<ll>(n,0));
    rep(i,n){
        rep(j,n){
            rep(k,n){
                c[i][j] ^= (a[i][k] & b[k][j]);
            }
        }
    }

    return c;
}

Matrix multi_re(Matrix a,ll n){
    int nn = a.size();
    Matrix ans(nn,vector<ll>(nn,0));

    ans = a;

    rep(i,nn) ans[i][i] = 1;
    while(n > 0){
        if(n & 1) {
            ans = multi(ans,a);
        }
        a = multi(a,a);
        n >>= 1;
    }

    return ans;
}



int main(){
    int n,k; cin >> n >> k;
    vi a(n); cin >> a;

    vector<double> s(n+1);
    vector<double> p(n);
    rep(i,n){
        int num = (a[i]*(a[i]+1))/2;
        p[i] = (num+0.0)/a[i];
    }

    rep(i,n){
        s[i+1] = s[i] + p[i];
    }

    double ans = 0;
    rep(i,n-k+1){
        ans = max(ans,s[i+k]-s[i]);
    }


    printf("%.9lf",ans);

}




Measured memory_bytes: 4096