#include <bits/stdc++.h>
// #pragma GCC optimize("Ofast")
// #pragma GCC target("avx,avx2,fma")
// #pragma GCC optimize("unroll-loops")
using namespace std;
typedef long long ll;

// region template
#pragma region template

#define pb push_back
#define eprintf(...) fprintf(stderr, __VA_ARGS__)

/**
 * \brief Input helpers.
 */
namespace In {

    template <class T> T in() {
        T val;
        cin >> val;
        return val;
    }

} // namespace In

/**
 * \brief Various useful helpers for STL.
 */
namespace STL {

/**
 * \brief Create a 2D vector<vector<T>> of dimension i * j.
 */
    template <class T> vector<vector<T>> vector2(int i, int j) { return vector<vector<T>>(i, vector<T>(j)); }

} // namespace STL

/**
 * \brief Various useful utilities for competitive programming.
 */
namespace Util {

// sprintf that returns a string.
    string ssprintf(const string fmt_str, ...) {
        int final_n, n = ((int)fmt_str.size()) * 2;
        unique_ptr<char[]> formatted;
        va_list ap;
        while (1) {
            formatted.reset(new char[n]);
            strcpy(&formatted[0], fmt_str.c_str());
            va_start(ap, fmt_str);
            final_n = vsnprintf(&formatted[0], n, fmt_str.c_str(), ap);
            va_end(ap);
            if (final_n < 0 || final_n >= n)
                n += abs(final_n - n + 1);
            else
                break;
        }
        return string(formatted.get());
    }

    template <class T> void assign(T &a, T b) { a = b; }
    template <class T, class U> void assign(T &a, U &b, T c, U d) { a = c, b = d; }
    template <class T, class U, class V> void assign(T &a, U &b, V &c, T d, U e, V f) { a = d, b = e, c = f; }
    template <class T, class U, class V, class W> void assign(T &a, U &b, V &c, W &d, T e, U f, V g, W h) {
        a = e, b = f, c = g, d = h;
    }

} // namespace Util

namespace Math {

    ll modpow(ll b, ll p, ll m) {
        ll r = 1;
        for (; p; p >>= 1, b = b * b % m)
            if (p & 1)
                r = r * b % m;
        return r;
    }

    ll modinv(ll b, ll m) { return modpow(b, m - 2, m); }

    ll sumdigit(ll num) {
        ll res = 0;
        while (num) {
            res += num % 10;
            num /= 10;
        }
        return res;
    }

    ll sumdigit(string num) {
        ll res = 0;
        for (char c : num)
            res += c - '0';
        return res;
    }

// Least significant bit.
    ll lsb(ll x) { return x & (-x); }

    template <class T> const T inf() { return 0; }
    template <> const ll inf<ll>() { return 0x3f3f3f3f3f3f3f3fll; }
    template <> const int inf<int>() { return 0x3f3f3f3f; }

    template <class T> T gcd(T a, T b) { return abs(__gcd(a, b)); }
    template <class T> T clamp(T n, T l, T r) { return n < l ? l : n > r ? r : n; }
    template <class T> T lcm(T a, T b) { return abs(a / gcd(a, b) * b); }
    template <class T> T sgn(T n) { return n == 0 ? 0 : n > 0 ? 1 : -1; }

    template <class T, int size> struct Vector;

// Matrix class.
    template <class T, int size_m, int size_n> struct Matrix {
        T val[size_m][size_n];

        Matrix() {
            for (int i = 0; i < size_m; i++)
                for (int j = 0; j < size_n; j++)
                    val[i][j] = 0;
        }

        /**
         * \brief Matrix type casting for another matrix of the same dimension.
         */
        template <class U> Matrix(Matrix<U, size_m, size_n> mat) {
            for (int i = 0; i < size_m; i++)
                for (int j = 0; j < size_n; j++)
                    val[i][j] = mat.val[i][j];
        }

        /**
         * \brief Construct a matrix from a 2D array.
         */
        Matrix(T **arr) {
            for (int i = 0; i < size_m; i++)
                for (int j = 0; j < size_n; j++)
                    val[i][j] = arr[i][j];
        }

        /**
         * \brief Returns the reference to the cell (i, j) in the matrix.
         */
        T &operator()(const int i, const int j) { return val[i][j]; }

        /**
         * \brief Explicitly cast the matrix to Matrix<U>.
         */
        template <class U> const Matrix<U, size_m, size_n> convert() { return Matrix<U, size_m, size_n>(*this); }

        const Matrix operator+=(const Matrix<T, size_m, size_n> mat) {
            for (int i = 0; i < size_m; i++)
                for (int j = 0; j < size_n; j++)
                    (*this)(i, j) += mat(i, j);
        }

        const Matrix operator-=(const Matrix<T, size_m, size_n> mat) {
            for (int i = 0; i < size_m; i++)
                for (int j = 0; j < size_n; j++)
                    (*this)(i, j) -= mat(i, j);
        }

        const Matrix operator+(const Matrix<T, size_m, size_n> mat) {
            Matrix<T, size_m, size_n> res = *this;
            res += mat;
            return res;
        }

        const Matrix operator-(const Matrix<T, size_m, size_n> mat) {
            Matrix<T, size_m, size_n> res = *this;
            res -= mat;
            return res;
        }

        /**
         * \brief Matrix-matrix multiplication.
         */
        template <int size_p> Matrix<T, size_m, size_p> operator*(Matrix<T, size_n, size_p> mat) {
            Matrix<T, size_m, size_p> res;
            for (int i = 0; i < size_m; i++)
                for (int j = 0; j < size_p; j++)
                    for (int k = 0; k < size_n; k++)
                        res(i, j) += (*this)(i, k) * mat(k, j);
            return res;
        }

        /**
         * \brief Matrix-vector multiplication.
         */
        Vector<T, size_n> operator*(Vector<T, size_n> vec) {
            Vector<T, size_n> res;
            for (int i = 0; i < size_m; i++)
                for (int j = 0; j < size_n; j++)
                    res.val[j] += val[i][j] * vec[j];
            return res;
        }
    };

// Vector class.
    template <class T, int size> struct Vector {
        /**
         * Specialization for Vector of size 2.
         */
        template <class U> static const Vector<U, 2> vec2(U a, U b) {
            Vector<U, 2> vec;
            vec.val[0] = a, vec.val[1] = b;
            return vec;
        }

        /**
         * Specialization for Vector of size 3.
         */
        template <class U> static const Vector<U, 3> vec3(U a, U b, U c) {
            Vector<U, 3> vec;
            vec.val[0] = a, vec.val[1] = b, vec.val[2] = c;
            return vec;
        }

        T val[size];

        /**
         * \brief Empty vector constructor.
         */
        Vector() {
            for (int i = 0; i < size; i++)
                val[i] = 0;
        }

        /**
         * \brief Vector type casting for another vector of the same size.
         */
        template <class U> Vector(Vector<U, size> vec) {
            for (int i = 0; i < size; i++)
                val[i] = vec.val[i];
        }

        /**
         * \brief Constructing a vector from an array.
         */
        Vector(T *arr) {
            for (int i = 0; i < size; i++)
                val[i] = arr[i];
        }

        /**
         * \brief Explicitly cast the vector to Vector<U> with the same size.
         */
        template <class U> Vector<U, size> convert() { return Vector<U, size>(*this); }

        const Vector operator+(const Vector &vec) const {
            Vector v = *this;
            v += vec;
            return v;
        }
        const Vector operator-(const Vector &vec) const {
            Vector v = *this;
            v -= vec;
            return v;
        }
        const Vector operator*(const Vector &vec) const {
            Vector v = *this;
            v *= vec;
            return v;
        }
        const Vector operator/(const Vector &vec) const {
            Vector v = *this;
            v /= vec;
            return v;
        }

        const Vector operator+=(const Vector &vec) {
            for (int i = 0; i < size; i++)
                val[i] += vec.val[i];
            return *this;
        }
        const Vector operator-=(const Vector &vec) {
            for (int i = 0; i < size; i++)
                val[i] -= vec.val[i];
            return *this;
        }
        const Vector operator*=(const Vector &vec) {
            for (int i = 0; i < size; i++)
                val[i] *= vec.val[i];
            return *this;
        }
        const Vector operator/=(const Vector &vec) {
            for (int i = 0; i < size; i++)
                val[i] /= vec.val[i];
            return *this;
        }

        /**
         * \brief Compute the dot product of two vectors.
         */
        const T dot(const Vector<T, size> vec) {
            T acc = 0;
            for (int i = 0; i < size; i++)
                acc += val[i] * vec.val[i];
            return acc;
        }
    };

    struct Log2Table {
        vector<int> val;
        // Last value that we computed to.
        int pmax = 0;

        Log2Table() {
            val.resize(1);
            val[0] = -1;
        }

        int get(int n) {
            if (n >= val.size()) {
                eprintf("Error: request log2[%d] but is not computed yet\n", n);
                exit(1);
            }

            return val[n];
        }

        void require(int n) {
            if (n < pmax)
                return;
            val.resize(n + 1);

            for (int i = 0; i < 30; i++)
                for (int j = max(1 << i, pmax + 1); j < min(1 << (i + 1), n + 1); j++)
                    val[j] = i;

            pmax = n;
        }
    };

    Log2Table log2t;

// Generic modular integer.
    template <int MOD, typename IntType = int> struct GModint {
        IntType val;

        GModint() : val(0) {}
        GModint(int v, bool safe = false) { val = safe ? v : ((v % MOD) + MOD) % MOD; }
        GModint(ll v, bool safe = false) { val = safe ? v : ((v % MOD) + MOD) % MOD; }

        explicit operator bool() const { return val != 0; }
        const bool operator==(const GModint &g) const { return val == g.val; }
        const bool operator!=(const GModint &g) const { return val != g.val; }
        const GModint operator+=(const GModint &g) { return *this = *this + g; }
        const GModint operator-=(const GModint &g) { return *this = *this - g; }
        const GModint operator*=(const GModint &g) { return *this = *this * g; }
        const GModint operator/=(const GModint &g) { return *this = *this / g; }
        const GModint operator^=(const GModint &g) { return *this = *this ^ g; }
        const GModint operator-() const { return GModint(val == 0 ? 0 : MOD - val, true); }
        const GModint operator+(const GModint &g) const {
            ll newval = val + g.val;
            if (newval >= MOD)
                newval -= MOD;
            return GModint(newval, true);
        }
        const GModint operator-(const GModint &g) const { return *this + (-g); }
        const GModint operator*(const GModint &g) const { return GModint(1ll * val * g.val); }
        const GModint operator/(const GModint &g) const { return *this * g.inv(); }
        template <class T> const GModint operator^(const T t) const { return GModint(modpow(val, t, MOD)); }
        const GModint inv() const { return GModint(modinv(val, MOD)); }
    };

    using Modint = GModint<1000000007>;

} // namespace Math

namespace DS {

// Building a 0-based sparse table for associative operations.
// i.e. A op (B op C) = (A op B) op C
    template <class T, class Combiner> struct SparseTable {
        int n, nlog, base;
        vector<vector<T>> st;
        Combiner combiner;

        SparseTable(int n, T *t, int base = 0) : n(n), base(base) {
            Math::log2t.require(n);

            nlog = Math::log2t.get(n);

            st.resize(nlog + 1);
            for (int i = 0; i <= nlog; i++)
                st[i].resize(n + base);

            for (int j = 0; j < n + base; j++)
                st[0][j] = t[j];
            for (int i = 1; i <= nlog; i++)
                for (int j = 0; j + (1 << (i - 1)) < n; j++)
                    st[i][j] = combiner(st[i - 1][j], st[i - 1][j + (1 << (i - 1))]);
        }

        T query(int l, int r) {
            if (l > r) {
                eprintf("Error: sparse table query (L, R) = (%d, %d).\n", l, r);
                exit(1);
            }

            l = Math::clamp(l, 0, n - 1);
            r = Math::clamp(r, 0, n - 1);

            int len = Math::log2t.get(r - l + 1);
            T val = st[len][l];
            l += 1 << len;

            if (l <= r)
                val = combiner(val, query(l, r));

            return val;
        }
    };

    template <class T> struct SparseTableCombinerSample {
        const T operator()(const T &a, const T &b) { return a + b; }
    };

    struct DSU {
        int size;
        vector<int> dsu, block;

        DSU() {}
        DSU(int size) { init(size); }

        void init(int new_size) {
            size = new_size;
            dsu.resize(size + 1);
            block.resize(size + 1);
            for (int i = 0; i <= size; i++)
                dsu[i] = i, block[i] = 1;
        }

        int find(int u) { return u == dsu[u] ? u : dsu[u] = find(dsu[u]); }
        bool same_component(int u, int v) { return find(u) == find(v); }
        void merge(int u, int v) {
            if (!same_component(u, v)) {
                block[find(u)] += block[find(v)];
                dsu[find(v)] = find(u);
            }
        }
        int block_size(int u) { return block[find(u)]; }
    };

} // namespace DS

namespace Graph {

    const int DEFAULT_NODE_LO = 1;
    const int NODE_NIL = -1;

    template <typename LenType> struct Edge {
        int from, to;
        LenType len;

        Edge(int from, int to, LenType len) : from(from), to(to), len(len) {}
    };

// The graph uses ID [lo, lo + n).
    template <typename LenType> struct Graph {
        using edge = Edge<LenType>;

        vector<vector<edge>> G;
        int n, lo;

        Graph() {}
        Graph(int size, int node_lo = DEFAULT_NODE_LO) { resize(size, node_lo); }

        void resize(int new_size, int node_lo = DEFAULT_NODE_LO) {
            n = new_size;
            lo = node_lo;
            G.resize(n + 1);
        }

        void add_edge(int from, int to, LenType len = 1) { G[from].push_back(edge(from, to, len)); }
        void add_bidir_edge(int u, int v, LenType len = 1) { add_edge(u, v, len), add_edge(v, u, len); }
    };

    template <typename LenType> struct BiColoring {
        bool success = true;
        vector<int> color;

        BiColoring(Graph<LenType> G) {
            color = vector<int>(G.n + 1, 0);

            queue<int> Q;
            Q.push(G.lo);
            color[G.lo] = 1;

            while (!Q.empty()) {
                int node = Q.front();
                Q.pop();
                for (Edge<LenType> edge : G.G[node]) {
                    if (color[edge.to] == color[edge.from]) {
                        success = false;
                        return;
                    }

                    if (!color[edge.to]) {
                        color[edge.to] = color[edge.from] != 1 ? 1 : 2;
                        Q.push(edge.to);
                    }
                }
            }
        }
    };

    template <typename LenType> struct FloydWarshall {
        vector<vector<LenType>> dist;

        FloydWarshall(Graph<LenType> G) {
            dist = vector<vector<LenType>>(G.n + 1, vector<LenType>(G.n + 1, Math::inf<LenType>()));

            for (int i = G.lo; i < G.lo + G.n; i++)
                dist[i][i] = 0;

            for (int i = G.lo; i < G.lo + G.n; i++)
                for (Edge<LenType> edge : G.G[i])
                    dist[edge.from][edge.to] = edge.len;

            for (int k = G.lo; k < G.lo + G.n; k++)
                for (int i = G.lo; i < G.lo + G.n; i++)
                    for (int j = G.lo; j < G.lo + G.n; j++)
                        dist[i][j] = min(dist[i][j], dist[i][k] + dist[k][j]);
        }
    };

    template <typename LenType> struct Tree : public Graph<LenType> {
        using edge = typename Graph<LenType>::edge;
        using Graph<LenType>::n;
        using Graph<LenType>::G;

        int root;
        vector<LenType> depth, parent;

        Tree() : Graph<LenType>() {}
        Tree(int size) : Graph<LenType>(size) {}

        void init_dfs(int n, int p = NODE_NIL) {
            typename vector<edge>::iterator it_parent = G[n].end();
            parent[n] = p;

            for (typename vector<edge>::iterator it = G[n].begin(); it != G[n].end(); it++) {
                if (it->to == p) {
                    it_parent = it;
                } else {
                    depth[it->to] = depth[n] + it->len;
                    init_dfs(it->to, n);
                }
            }
        }

        void init_tree(int new_root = 1, int root_depth = 0) {
            root = new_root;

            depth.resize(n + 1);
            parent.resize(n + 1);

            depth[root] = root_depth;

            init_dfs(root);
        }
    };

    using UnweightedTree = Tree<int>;

    struct LCA {
        int n, nlog;
        vector<int> depth;
        vector<vector<int>> jump_table;

        LCA(const UnweightedTree &tree) { init(tree); }
        void init(const UnweightedTree &tree) {
            n = tree.n;
            depth = tree.depth;

            Math::log2t.require(n);
            nlog = Math::log2t.get(n);

            jump_table.resize(nlog + 1);
            for (int i = 0; i <= nlog; i++)
                jump_table[i].resize(n + 1);
            for (int i = 0; i <= n; i++)
                jump_table[0][i] = tree.parent[i];
            for (int i = 1; i <= nlog; i++)
                for (int j = 0; j <= n; j++)
                    jump_table[i][j] = jump_table[i - 1][j] == NODE_NIL ? NODE_NIL : jump_table[i - 1][jump_table[i - 1][j]];
        }

        int up(int node_id, int level) {
            for (int i = nlog; i >= 0; i--)
                if ((level >> i) & 1)
                    node_id = jump_table[i][node_id];
            return node_id;
        }

        int lca(int u, int v) {
            if (depth[u] > depth[v])
                swap(u, v);
            v = up(v, depth[v] - depth[u]);
            if (u == v)
                return u;
            for (int i = nlog; i >= 0; i--)
                if (jump_table[i][u] != jump_table[i][v])
                    u = jump_table[i][u], v = jump_table[i][v];
            return jump_table[0][u];
        }

        int dist(int u, int v) {
            int l = lca(u, v);
            return depth[u] + depth[v] - 2 * depth[l];
        }

        int furthest_node(int node, int node_lo, int node_hi) {
            int furthest = node, furthest_dist = 0;
            for (int i = node_lo; i <= node_hi; i++)
                if (i != node) {
                    int new_dist = dist(node, i);
                    if (new_dist > furthest_dist)
                        furthest = i, furthest_dist = new_dist;
                }
            return furthest;
        }

        int diameter(int node_lo, int node_hi) {
            int node1 = furthest_node(node_lo, node_lo, node_hi);
            int node2 = furthest_node(node1, node_lo, node_hi);
            return dist(node1, node2);
        }
    };

} // namespace Graph

namespace Regex {

    bool match(string regex_rule, string content) {
        regex r(regex_rule);
        return regex_match(content, r);
    }

} // namespace Regex

template <int MOD> ostream &operator<<(ostream &out, const Math::GModint<MOD> m) {
    out << m.val;
    return out;
}

template <class T> ostream &operator<<(ostream &out, const vector<T> v) {
    out << "[";
    for (typename vector<T>::iterator it = v.begin(); it != v.end(); it++) {
        out << *it;
        if (next(it) != v.end())
            out << ", ";
    }
    out << "]";
    return out;
}

template <class T, class U> ostream &operator<<(ostream &out, const map<T, U> m) {
    out << "{";
    for (typename map<T, U>::const_iterator it = m.begin(); it != m.end(); it++) {
        out << it->first << " -> " << it->second;
        if (next(it) != m.end())
            out << ", ";
    }
    out << "}";
    return out;
}

template <class T> ostream &operator<<(ostream &out, const set<T> s) {
    out << "(";
    for (typename set<T>::iterator it = s.begin(); it != s.end(); it++) {
        out << *it;
        if (next(it) != s.end())
            out << ", ";
    }
    out << ")";
    return out;
}

template <class T, class U> ostream &operator<<(ostream &out, const pair<T, U> p) {
    out << "(" << p.first << ", " << p.second << ")";
    return out;
}

// endregion
#pragma endregion template

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);

    int n, k; cin >> n >> k;
    vector<long long> vd(n);
    for (int i = 0; i < n; i++) {
        cin >> vd[i];
        vd[i]++;
    }
    vector<long long> ps(n);
    partial_sum(vd.begin(), vd.end(), ps.begin());
    long long maxsum = 0;
    maxsum = ps[k - 1];
    for (int i = k; i < n; i++) {
        maxsum = max(maxsum, ps[i] - ps[i - k]);
    }
    cout << fixed << setprecision(10) << (1.0 * maxsum / 2) << endl;

}


Measured memory_bytes: 3456