#include <bits/stdc++.h>
using namespace std;
#define ll unsigned long long int
#define mod 1000000007
bool isPrime[1000001];
ll power(ll a,ll b)
{
    ll result = 1;
    while(b>0)
    {
        if(b%2)
        {
            result = (a*result)%mod;
        }
        a = (a*a)%mod;
        b = b/2;
    }
    return result;
}
int gcd(int a,int b)
{
    if(b==0)
    {
        return a;
    }
    return gcd(b,a%b);
}
void sieve()
{
    memset(isPrime,1,sizeof(isPrime));
    isPrime[0] = isPrime[1] = 0;
    for(int i=2;i*i<=1000000;i++)
    {
        if(isPrime[i])
        {
            for(int j=i*i;j<=1000000;j+=i)
            {
                isPrime[j] = 0;
            }
        }
    }
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL),cout.tie(NULL);
    int t,n;
    t = 1;
    for(int i=0;i<t;i++)
    {
        int n,k;
        cin>>n>>k;
        double arr[n];
        for(int i=0;i<n;i++)
        {
            cin>>arr[i];
            arr[i] = (arr[i]+1)/2;
        }
        double sum = 0;
        for(int i=0;i<k;i++)
        {
            sum+=arr[i];
        }
        double maxi = sum;
        for(int i=k;i<n;i++)
        {
            sum+=arr[i];
            sum-=arr[i-k];
            maxi = max(sum,maxi);
        }
        cout<<fixed<<setprecision(15)<<maxi;
    }
    return 0;
}

#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define all(a) a.begin(), a.end()
#define pb push_back
#define mp make_pair
typedef long long ll;
typedef pair<ll,ll> P;
typedef pair<ll,int> pli;
typedef pair<int,int> pii;
#define rep(i,a,b) for(ll i=a ; i<b ; i++)
#define qrep(que, ite) for(auto ite=begin(que) ; ite!=end(que) ; ite++)
const int max_n = 1e5;
const ll mod = 1e9+7;
const ll INF = 1e17;
const int inf = 1e5;
typedef long double ld;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
ll gcd(ll a, ll b) { return a ? gcd(b%a, a) : b; }
int dx[4] = {0,0,1,-1};
int dy[4] = {1,-1,0,0};
struct UnionFind {
    vector<int> par;

    UnionFind(int n) : par(n, -1) { }
    void init(int n) { par.assign(n, -1); }

    int root(int x) {
        if (par[x] < 0) return x;
        else return par[x] = root(par[x]);
    }

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

    bool merge(int x, int y) {
        x = root(x); y = root(y);
        if (x == y) return false;
        if (par[x] > par[y]) swap(x, y); // merge technique
        par[x] += par[y];
        par[y] = x;
        return true;
    }

    int size(int x) {
        return -par[root(x)];
    }
};
struct SegmentTree{
  int N;
  vector<int> node;
  public :
    void intit(vector<int>v){
      int sz = v.size();
      N=1;
      while(N<sz) N*=2;
      node.resize(N);
      for(int i=0 ; i<sz ; i++) node[i+N-1] = v[i];
      for(int i=N-2 ; i>=0 ; i--) node[i] = min(node[i*2+1], node[i*2+2]);
    }
    void update(int x, int val){
      x += N-1;
      node[x+N-1] = val;
      while(x>0){
        x = (x-1)/2;
        node[x] = min(node[x*2+1], node[x*2+2]);
      }
    }
    int getmin(int a, int b, int k, int l, int r){
      if(b<=l || r<=a) return inf;
      else if(a<=l && r<=b) return node[k];
      else{
        int vl = getmin(a, b, 2*k+1, l, (l+r)/2);
        int vr = getmin(a, b, 2*k+2, (l+r)/2, r);
        return min(vl, vr);
      }
    }
};
int main(){
  ll N,K; cin>>N>>K;
  vector<ll> v(N);
  rep(i,0,N){
    cin >> v[i];
    v[i]++;
  }
  vector<ll> v2(N+1, 0);
  rep(i,0,N) v2[i+1] = v2[i] + v[i];
  ll res = 0;
  for(ll i=0 ; i+K<=N ; i++){
    chmax(res, v2[i+K] - v2[i]);
  }
  printf("%.7f", (double)res/2);
  return 0;
}
