#pragma region template 2.4
#include <bits/stdc++.h>
using namespace std;
template <typename T>
using pq_asc = priority_queue<T, vector<T>, greater<T>>;
typedef long long ll;
typedef vector<ll> vi;
typedef vector<vi> vvi;
typedef pair<ll, ll> ii;
typedef vector<ii> vii;
typedef vector<string> vs;
#define REP(i, n) for (ll i = 0; i < (n); ++i)
#define REP1(i, n) for (ll i = 1; i <= (n); ++i)
#define FOR(i, a) for (auto &i : a)
#define CH(f, x, y) x = f(x, y)
#define IN(T, x) \
    T x;         \
    cin >> x;
#define AIN(T, a, n) \
    vector<T> a(n);  \
    FOR(i, a)        \
    cin >> i;
#define A2IN(T1, a, T2, b, n) \
    vector<T1> a(n);          \
    vector<T2> b(n);          \
    REP(i, n)                 \
    cin >> a[i] >> b[i];
#define OUT(x) cout << (x) << endl;
#define FOUT(x) cout << fixed << setprecision(15) << (x) << endl;
#define ALL(a) (a).begin(), (a).end()
#define SORT(a) sort(ALL(a))
#define RSORT(a) \
    SORT(a);     \
    reverse(ALL(a))
#define DUMP(x) cout << #x << " = " << (x) << endl;
#define DUMPA(a)          \
    cout << #a << " = {"; \
    JOUT(ALL(a), ", ", cout) << "}" << endl;

template <typename T>
ostream &JOUT(T s, T e, string sep = " ", ostream &os = cout)
{
    if (s != e)
    {
        os << *s;
        ++s;
    }
    while (s != e)
    {
        os << sep << *s;
        ++s;
    }
    return os;
}

ostream &YES(bool cond, string yes = "Yes", string no = "No", ostream &os = cout)
{
    if (cond)
    {
        os << yes << endl;
    }
    else
    {
        os << no << endl;
    }
    return os;
}

template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p)
{
    os << '(' << p.first << ", " << p.second << ')';
    return os;
}

template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v)
{
    os << '[';
    JOUT(ALL(v), ", ", os) << ']';
    return os;
}

const ll INF = 1e18;
const ll MOD = 1e9 + 7;
#pragma endregion template

int main()
{
    IN(ll, N);
    IN(ll, K);
    AIN(ll, p, N);
    double s = 0;
    REP(i, K)
    {
        s += (1 + p[i]) / 2.0;
    }
    double ans = s;
    for (ll i = K; i < N; ++i)
    {
        s += (p[i] - p[i - K]) / 2.0;
        CH(max, ans, s);
    }
    FOUT(ans);
}
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

using namespace std;
using ll = long long;

double expected_value(int n) {
  return (double)((1 + n) * n / 2) / (double)n;
}

int main() {
  int n, k;
  cin >> n >> k;
  vector<double> ps(n + 1);
  vector<double> comsum(n + 1, 0);
  int p;
  rep(i, n) {
    cin >> p;
    ps[i + 1] = expected_value(p);
    comsum[i + 1] = comsum[i] + ps[i + 1];
  }
  double mm = 0;
  for (int i = k; i <= n; i++) {
    mm = max(comsum[i] - comsum[i - k], mm);
  }
  printf("%.6f\n", mm);
  return 0;
}
