#include <bits/stdc++.h>

using namespace std;

using ll = long long;
using ull = unsigned long long;
using vll = vector<ll>;
using vi = vector<int>;
using vb = vector<bool>;
using pll = pair<ll, ll>;
using pii = pair<int, int>;
using vpii = vector<pii>;
using vpll = vector<pll>;
const ll LINF = 1ll << 55;
const ll INF = 0x3f3f3f3f;
const ll MOD = 1e9 + 7;
const int MAXN = 1e5 + 10;
const ll dx[] = {1, 0, -1, 0};
const ll dy[] = {0, 1, 0, -1};

/// cin/cout overloading
template <typename T>
ostream &operator<<(ostream &out, vector<T> &vec) {
    for (auto it = vec.begin(); it != vec.end(); ++it) {
        out << *it << " ";
    }
    return out;
}

template <typename T>
ostream &operator<<(ostream &out, pair<T, T> &P) {
    out << P.first << " " << P.second;
    return out;
}

template <typename T>
istream &operator>>(istream &in, vector<T> &vec) {
    for (auto it = vec.begin(); it != vec.end(); ++it) {
        in >> *it;
    }
    return in;
}

template <typename T>
istream &operator>>(istream &in, pair<T, T> &P) {
    in >> P.first >> P.second;
    return in;
}

#define coutp(x, y) cout << (x) << " " << (y) << endl
#define coutn(x) cout << (x) << endl
#define coutd(x) cout << setprecision(10) << (x) << endl

/// 图相关
// ll N, M;

// struct Edge {
//     int to;
//     ll weight;
//     int next;
//     bool operator<(const Edge &rhs) { return weight < rhs.weight; }
// };

// template <typename T>
// struct Edge2 {
//     T from, to, weight;
//     Edge2(T from_, T to_, T weight_) : from(from_), to(to_), weight(weight_)
//     {} Edge2() : from(T(0)), to(T(0)), weight(T(0)) {} bool operator<(const
//     Edge2 &rhs) { return weight < rhs.weight; }
// };

// vector<Edge> edges(MAXN << 1);
// vi head(MAXN, -1);
// vector<int> matchingx(MAXN, -1);
// vector<int> matchingy(MAXN, -1);
// vector<bool> check(MAXN);
// vector<ll> dis(MAXN);
// vector<bool> vis(MAXN, false);

// int cnt = 1;

// void addEdge(int from, int to, ll weight) {
//     edges[cnt].to = to;
//     edges[cnt].weight = weight;
//     edges[cnt].next = head[from];
//     head[from] = cnt++;
// }

// bool dfs(int u) {
//     for (int i = head[u]; i != -1; i = edges[i].next) {
//         int v = edges[i].to;
//         if (!check[v]) {
//             check[v] = true;
//             if (matchingy[v] == -1 || dfs(matchingy[v])) {
//                 matchingy[v] = u;
//                 matchingx[u] = v;
//                 return true;
//             }
//         }
//     }
//     return false;
// }

// int hungarian() {
//     int ans = 0;
//     fill(matchingx.begin(), matchingx.end(), -1);
//     fill(matchingy.begin(), matchingy.end(), -1);
//     for (int u = 1; u <= N; ++u) {
//         //        if (matchingx[u] == -1) {
//         {
//             fill(check.begin(), check.end(), false);
//             if (dfs(u)) {
//                 ++ans;
//             }
//         }
//     }
//     return ans;
// }

// void dijkstra(const ll s) {
//     priority_queue<P, vector<P>, greater<P>> que;
//     fill(dis.begin(), dis.end(), INF);
//     dis[s] = 0;
//     que.push(P(0, s));
//     // multiple sources
//     for (auto &x : shops) {
//         dis[x] = 0;
//         que.push(P(0, x));
//     }
//     while (!que.empty()) {
//         P p = que.top();
//         que.pop();
//         cout << "pop " << p.second << endl;
//         int u = p.second;
//         if (dis[u] < p.first) continue;
//         for (int i = head[u]; i != -1; i = edges[i].next) {
//             int v = edges[i].to;
//             if (dis[v] > dis[u] + edges[i].weight) {
//                 dis[v] = dis[u] + edges[i].weight;
//                 cout << "push " << v << endl;
//                 que.push(P(dis[v], v));
//             }
//         }
//     }
// }

// void zeroOneBFS(const int s) {
//     deque<P> que;
//     fill(dis.begin(), dis.end(), INF);
//     dis[s] = 0;
//     que.push_front(P(0, s));
//     while (!que.empty()) {
//         P p = que.front();
//         que.pop_front();
//         int u = p.second;
//         if (dis[u] < p.first) continue;
//         for (int i = head[u]; i != -1; i = edges[i].next) {
//             int v = edges[i].to;
//             if (dis[v] > dis[u] + edges[i].weight) {
//                 dis[v] = dis[u] + edges[i].weight;
//                 if (edges[i].weight) {
//                     que.push_back(P(dis[v], v));
//                 } else {
//                     que.push_front(P(dis[v], v));
//                 }
//             }
//         }
//     }
// }

/// Union-Find 并查集
class UnionFind {
    vector<ll> par;

   public:
    explicit UnionFind(ll n) : par(n, -1) {}

    ll root(ll a) {
        if (par[a] < 0) {
            return a;
        }
        return par[a] = root(par[a]);
    }

    ll size(ll a) { return -par[root(a)]; }

    void unite(ll a, ll b) {
        a = root(a);
        b = root(b);
        if (a != b) {
            if (size(a) < size(b)) {
                swap(a, b);
            }
            par[a] += par[b];
            par[b] = a;
        }
    }
};

// template <typename T>
// ll kruskal(vector<Edge2> &edges2, const ll V) {
//     sort(edges2.begin(), edges2.end());
//     UnionFind uf(V + 10);
//     ll res = 0;
//     Edge2 e;
//     for (int i = 0; i < edges2.size(); ++i) {
//         e = edges2[i];
//         if (uf.root(e.u) != uf.root(e.v)) {
//             uf.unite(e.u, e.v);
//             res += e.w;
//         }
//     }
//     return res;
// }

/// 线段树
struct SegmentTreeNode {
    ll maxVal;
    ll minVal;
    ll sum;
    ll len;
    ll lazy;
    ll left, right;
    SegmentTreeNode() {}
};

class SegmentTree {
   public:
    explicit SegmentTree(size_t size, const vll &nums)
        : tree(size << 2), nums(nums) {}
    void build(ll root, ll left, ll right) {
        tree[root].lazy = 0;
        tree[root].left = left;
        tree[root].right = right;
        tree[root].len = right - left + 1;
        if (left == right) {
            tree[root].maxVal = nums[left];
            tree[root].minVal = nums[left];
            tree[root].sum = nums[left];
        } else {
            ll mid = (right - left) / 2 + left;
            build(root * 2, left, mid);
            build(root * 2 + 1, mid + 1, right);
            tree[root].minVal =
                min(tree[root * 2].minVal, tree[root * 2 + 1].minVal);
            tree[root].maxVal =
                max(tree[root * 2].maxVal, tree[root * 2 + 1].maxVal);
            tree[root].sum = tree[root * 2].sum + tree[root * 2 + 1].sum;
        }
    }

    void pushup(ll root) {
        tree[root].minVal =
            min(tree[root * 2].minVal, tree[root * 2 + 1].minVal);
        tree[root].maxVal =
            max(tree[root * 2].maxVal, tree[root * 2 + 1].maxVal);
        tree[root].sum = tree[root * 2].sum + tree[root * 2 + 1].sum;
    }
    // add single node val
    void add(ll root, ll id, ll addVal) {
        if (tree[root].left == tree[root].right) {
            tree[root].maxVal += addVal;
            tree[root].minVal += addVal;
            tree[root].sum += addVal;
            return;
        }
        ll mid = (tree[root].right - tree[root].left) / 2 + tree[root].left;
        if (id <= mid) {
            add(root * 2, id, addVal);
        } else {
            add(root * 2 + 1, id, addVal);
        }
        pushup(root);
    }

    void pushdown(ll root) {
        if (tree[root].lazy) {
            tree[root * 2].lazy += tree[root].lazy;
            tree[root * 2 + 1].lazy += tree[root].lazy;
            tree[root * 2].sum += tree[root * 2].len * tree[root].lazy;
            tree[root * 2 + 1].sum += tree[root * 2 + 1].len * tree[root].lazy;
            tree[root * 2].maxVal += tree[root].lazy;
            tree[root * 2 + 1].maxVal += tree[root].lazy;
            tree[root * 2].minVal += tree[root].lazy;
            tree[root * 2 + 1].minVal += tree[root].lazy;
            tree[root].lazy = 0;
        }
    }
    // query range sum
    ll querySum(ll root, ll left, ll right) {
        if (tree[root].left >= left && tree[root].right <= right) {
            return tree[root].sum;
        }
        if (tree[root].left > right || tree[root].right < left) {
            return 0;
        }
        if (tree[root].lazy) {
            pushdown(root);
        }
        return querySum(root * 2, left, right) +
               querySum(root * 2 + 1, left, right);
    }
    // query range max/min
    ll queryMax(ll root, ll left, ll right) {
        if (tree[root].left >= left && tree[root].right <= right) {
            return tree[root].maxVal;
        }
        if (tree[root].left > right || tree[root].right < left) {
            return -INF;
        }
        if (tree[root].lazy) {
            pushdown(root);
        }
        return max(queryMax(root * 2, left, right),
                   queryMax(root * 2 + 1, left, right));
    }

    ll queryMin(ll root, ll left, ll right) {
        if (tree[root].left >= left && tree[root].right <= right) {
            return tree[root].minVal;
        }
        if (tree[root].left > right || tree[root].right < left) {
            return INF;
        }
        if (tree[root].lazy) {
            pushdown(root);
        }
        return min(queryMin(root * 2, left, right),
                   queryMin(root * 2 + 1, left, right));
    }
    // add range val
    void update(ll root, ll left, ll right, ll addVal) {
        if (tree[root].left >= left && tree[root].right <= right) {
            tree[root].lazy += addVal;
            tree[root].sum += tree[root].len * addVal;
            tree[root].maxVal += addVal;
            tree[root].minVal += addVal;
            return;
        }
        if (tree[root].left > right || tree[root].right < left) {
            return;
        }
        if (tree[root].lazy) {
            pushdown(root);
        }
        update(root * 2, left, right, addVal);
        update(root * 2 + 1, left, right, addVal);
        pushup(root);
    }

   private:
    vector<SegmentTreeNode> tree;
    const vll &nums;
};

/// 组合数

/// 約数求める
// void divisor(ll n, vector<ll> &ret) {
//     for (ll i = 1; i * i <= n; i++) {
//         if (n % i == 0) {
//             ret.push_back(i);
//             if (i * i != n) ret.push_back(n / i);
//         }
//     }
//     sort(ret.begin(), ret.end());
// }
/// 階乗
ll factorial(ll n) {
    ll ret = 1;
    for (ll i = 1; i <= n; ++i) {
        ret = (ret * i) % MOD;
    }
    return ret;
}
/// モジュラ逆数
ll inv_mod(ll n) {
    ll a = n % MOD, b = MOD - 2, ret = 1;
    while (b > 0) {
        if (b & 1) ret = (ret * a) % MOD;
        a = (a * a) % MOD;
        b >>= 1;
    }
    return ret;
}

ll nPr(ll n, ll r) {
    ll ret = 1;
    for (ll i = n; i >= n - r + 1; --i) {
        ret = ret * (i % MOD) % MOD;
    }
    return ret;
}

ll nCr(ll n, ll r) {
    if (r > n - r) {
        return nCr(n, n - r);
    }
    return nPr(n, r) * inv_mod(factorial(r)) % MOD;
}
//
// vll F(MAXN), Finv(MAXN), inv(MAXN);

// ll pow_mod(ll a, ll b, ll p) {
//     ll ret = 1;
//     while (b) {
//         if (b & 1) ret = (ret * a) % p;
//         a = (a * a) % p;
//         b >>= 1;
//     }
//     return ret;
// }

// void comb_init() {
//     inv[1] = 1;
//     for (int i = 2; i < MAXN; ++i) {
//         inv[i] = (MOD - MOD / i) * 1ll * inv[MOD % i] % MOD;
//     }
//     F[0] = Finv[0] = 1;
//     for (int i = 1; i < MAXN; ++i) {
//         F[i] = F[i - 1] * 1ll * i % MOD;
//         Finv[i] = Finv[i - 1] * 1ll * inv[i] % MOD;
//     }
// }

// inline ll comb(ll n, ll m) {
//     if (m < 0 || m > n) return 0;
//     return F[n] * 1ll * Finv[n - m] % MOD * Finv[m] % MOD;
// }
//
inline ll ADD(ll a, ll b) { return (a + b) % MOD; }

inline ll MUL(ll a, ll b) { return (a % MOD) * (b % MOD) % MOD; }

inline ll SUB(ll a, ll b) { return (a + MOD - b) % MOD; }

inline ll POW(ll a, ll b) {
    ll ret = 1;
    while (b) {
        if (b & 1) ret = (ret * a) % MOD;
        a = (a * a) % MOD;
        b >>= 1;
    }
    return ret;
}

/// main函数
int main() {
    int N, K;
    cin >> N >> K;
    vi nums(N);
    cin >> nums;
    ll sum = 0;
    ll maxSum = 0;
    for (int i = 0; i < K; ++i) {
        sum += nums[i];
    }
    maxSum = sum;
    for (int i = K; i < N; ++i) {
        sum -= nums[i - K];
        sum += nums[i];
        maxSum = max(maxSum, sum);
    }
    double ans = K * 1.0 / 2 + 0.5 * maxSum;
    cout << setprecision(10) << ans << endl;
    return 0;
}

Measured memory_bytes: 1024