//#pragma comment(linker, "/stack:200000000") //  (UNCOMMENT WHEN HAVING LOTS OF RECURSIONS)
#pragma GCC optimize("O3")
#pragma GCC optimize("Ofast")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
//#pragma GCC optimize("unroll-loops")  // $ (UNCOMMENT WHEN TRYING TO BRUTEFORCE WITH A LOT OF LOOPS)\
 
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
 
#define fast_cin ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define endl "\n"
#define int long long
#define pb push_back
#define all(con) (con).begin(), (con).end()
#define fi first
#define se second
#define rep(i,a,b) for(int i=a;i<b;++i)
#define fre(i,a,b) for(int i=a;i<=b;i++)
#define rpe(i,a,b) for(i=a-1;i>=b;i--)
#define rp(i,n) rep(i,0,n)
#define fr(i,n) fre(i,1,n)
#define ff(i,a,b) for(int i=a;i<=b;i++)
 
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef vector<int> vi;
 
template <typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
// order_of_key (val): returns the no. of values less than val
// find_by_order (k): returns the iterator to kth largest element.(0-based)
 
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
 
#define TRACE
#ifndef ONLINE_JUDGE
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1>
void __f(const char* name, Arg1&& arg1){
	cerr << name << " : " << arg1 << endl;}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args){
	const char* comma = strchr(names + 1, ',');cerr.write(names, comma - names) << " : " << arg1<<"  ";__f(comma+1, args...);}
#else
#define trace(...)
#endif
 
template<class S,class T> ostream& operator<<(ostream& o,const pair<S,T> &p){
	return o<<"("<<p.fs<<","<<p.sc<<")";
}
template<class T> ostream& operator<<(ostream& o,const vector<T> &vc){
	o<<"{";
	for(const T& v:vc) o<<v<<",";
	o<<"}";
	return o;
}
 
int modpow(int x, int n, int m) {
	x %= m;
	int res = 1;
	while(n > 0) {
		if(n & 1)
		res = res * x % m;
		x = x * x % m;
		n >>= 1;
	}
	return res;
}
 
const long long inf = 2e18 ;
const int mod = 1e9+7 ;
// const int mod = 998244353 ;
 
const int N = 1e5 + 10 ;
 
long double f(int x) {
        return (1.0*(x+1)/2);
}

void solve() {
        int n,k;
        cin>>n>>k;
        int a[n];
        for(int i=0;i<n;i++) {
                cin>>a[i];
        }
        int ans=0;
        for(int i=0;i<k;i++) {
                ans+=a[i];
        }
        int cur=ans;
        // trace(ans);
        for(int i=1;i<=n-k;i++) {
                int tmp=cur;
                tmp-=a[i-1];
                // trace(a[i-1]);
                // trace(tmp);
                tmp+=a[i+k-1];
                // trace(a[i+k-1]);
                // trace(tmp);
                ans=max(ans,tmp);
                // trace(ans,tmp);
                cur=tmp;
        }
        ld res=ans+k;
        res=1.0*res/2;
        cout<<setprecision(10);
        cout<<res<<endl;
        // ld ans=0;
        // for(int i=0;i<k;i++) {
        //         ans+=f(a[i]); 
        // }
        // ld cur=ans;
        // trace(ans);
        // for(int i=1;i<=n-k;i++) {
        //         trace(ans);
        //         ld tmp=cur;
        //         tmp-=f(a[i-1]); 
        //         tmp+=f(a[i]);
        //         ans=max(ans,tmp); 
        //         cur=tmp;
        // }
        // cout<<ans<<endl;
}
 
signed main() {
        fast_cin;
	
	srand(chrono::high_resolution_clock::now().time_since_epoch().count());
        #ifndef ONLINE_JUDGE
        // freopen("input.txt", "r", stdin);
        // freopen("output.txt", "w", stdout);
    	#endif
	
	int tests=1;
	// cin>>tests;
	while(tests--) {
		solve();
	}
 
return 0 ;
}
 

Measured memory_bytes: 1792