#include <algorithm>
#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>
#include <bitset>
#include <deque>
#include <queue>
#include <stack>
#include <list>
#include <map>
#include <set>
#include <cmath>
#include <limits>
#include <random>
#include <numeric>
#include <iomanip>
#include <cstdlib>
#include <iterator>
#include <functional>
#include <unordered_map>
#include <unordered_set>

using namespace std;

typedef unsigned                    ui;
typedef int                         si;
typedef long                        sl;
typedef long long                   ll;
typedef unsigned long long          ull;

// ----------------

template <typename A, typename B>
string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string& s) {
  return '"' + s + '"';
}

string to_string(const char* s) {
  return to_string((string) s);
}

string to_string(const char ch) {
    string str;
    str.push_back('\'');
    str.push_back(ch);
    str.push_back('\'');
    return str;
}

string to_string(bool b) {
  return (b ? "true" : "false");
}

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}

template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

// ----------------

bool Random() {
    std::random_device r;
    std::default_random_engine e1(r());
    std::uniform_int_distribution<int> n_dist(1, 5);

    ll n = n_dist(e1);

    return true;
}

int main(int argc, char *argv[])
{
    std::ios_base::sync_with_stdio(false);

    ll k, n;
    cin >> n >> k;

    vector<ll> a(n, 0);
    for (int i = 0; i < n; ++i) {
        cin >> a[i];
    }

    const auto get_p = [](ll x) -> double {
        ll c = (x + 1) * x;
        c /= 2;

        return c * (static_cast<double>(1) / x);
    };

    vector<double> cs(n + 1, 0);
    for (int i = 0; i < n; ++i) {
        cs[i + 1] = cs[i] + get_p(a[i]);
    }

    debug(a);
    debug(cs);

    double max_sum = 0;
    for (int i = 0; i + k <= n; ++i) {
        double sum = cs[i + k] - cs[i];
        max_sum = max(max_sum, sum);
    }

    debug(max_sum);

    cout << fixed << setprecision(12) << (static_cast<double>(max_sum)) << endl;

    return 0;
}


Measured memory_bytes: 3456