#include <stdio.h>
#include <iostream>
#include <algorithm>
#include <sstream>
#include <complex>
#include <vector>
#include <list>
#include <queue>
#include <deque>
#include <stack>
#include <set>
#include <map>
#include <cstdio>
#include <string>
#include <cstdlib>
#include <cstring>
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef string str;
typedef std::pair<ll,ll> pl;
typedef std::map<string, ll> msl;
typedef std::map<char, ll> mcl;
typedef std::map<ll, ll> mll;
typedef std::vector<ll> vl;
typedef std::vector<pl> vpl;
const ll MOD = 1e9 + 7;
const ll ZERO = 0;
const ll INF = MOD * MOD;
const long double EPS = 1e-9;

#define rep(i, n) for(ll (i) = 0; (i) < (n) ; (i)++ )
#define revrep(i, n) for(ll (i) = n - 1; (i) >= 0 ; (i)--)
#define For(i, a, b) for(ll (i) = (a); (i) < (b) ; (i)++ )
#define revFor(i, b, a) for(ll (i) = (b) - 1; (i) >= (a) ; (i)--)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define isUpper(c) ('a' - c > 0)
#define toLower(c) char((c)+0x20)
#define toUpper(c) char((c)-0x20)
#define pb push_back
#define mp make_pair
#define pr(a) cout << (a)
#define prl(a) cout << (a) << endl
#define prl2(a,b) cout << (a) << " " << (b) << endl
#define prs(a) cout << (a) << " "
#define yn(condition) if ((condition)) prl("Yes"); else prl("No");
#define YN(condition) if ((condition)) prl("YES"); else prl("NO");
#define rpr(i,n,ans) rep((i),(n)) {if ((i) < n - 1) prs((ans[(i)])); else if ((i) == n - 1) prl((ans[(i)]));}
#define cpr(i,n,ans) rep((i),(n)) prl((ans[(i)]))
#define in1(a) cin >> (a)
#define in2(a,b) cin >> (a) >> (b)
#define in3(a,b,c) cin >> (a) >> (b) >> (c)
#define in4(a,b,c,d) cin >> (a) >> (b) >> (c) >> (d)
#define rin1(i,n,a) rep((i),(n)) cin >> (a[(i)])
#define rin2(i,n,a,b) rep((i),(n)) cin >> (a[(i)]) >> (b[(i)])
#define rin3(i,n,a,b,c) rep((i),(n)) cin >> (a[(i)]) >> (b[(i)]) >> (c[(i)])
#define matin(i,h,j,w,a) rep((i),(h)) rep((j),(w)) cin >> a[i][j]
#define e1 first
#define e2 second
#define ctol(c) ll((c)) - ll('0')
#define ltos(n) to_string((n))
#define items(kv, v) for(auto &(kv): (v))
#define ndig(N, n) ctol(ll(ltos((N))[ll(ltos((N)).length()) - (n)]))
#define beto(a,b,c) (a) < (b) && (b) < (c)
#define betc(a,b,c) (a) <= (b) && (b) <= (c)
#define rsort(a,n) sort(a,a+n,greater<>())
#define Forchar(c,a,z) for(char (c) = (a); (c) <= (z); (c)++)
#define cntchar(s,c) count(all((s)), c)
#define substring(s, start, end) s.substr((start), (end) - (start) + 1)
#define prl_nd(num, digits) cout << fixed << setprecision(digits) << (num) << endl;
#define XOR(a, b) (a)^(b)

ll gcd(ll m, ll n){
    ll a = max(m, n);
    ll b = min(m, n);
    while(b != 1 && b != 0){ a %= b; swap(a, b);}
    return b == 1 ? 1 : a;
}

ll lcm(ll m, ll n){ return m / gcd(m, n) * n;}

ll power_mod(ll a, ll power, ll mod){
    ll value = 1;
    while(power != 0){
        if(power & 1) value = (value * a) % mod;
        a = (a * a) % mod;
        power = power >> 1;
    }
    return value % mod;
}

ll modinv(ll a, ll mod){
    return power_mod(a, mod - 2, mod);
}

ll power_normal(ll a, ll power){
    ll value = 1;
    while(power != 0){
        if(power & 1) value = value * a;
        a = a * a;
        power = power >> 1;
    }
    return value;
}

ll combination(ll n, ll r){
    ll numerator = 1;
    ll denomenator = 1;
    for(ll i = 0; i < r ; i++){
        (numerator *= ll(n - i)) %= MOD;
        (denomenator *= modinv(i + 1, MOD)) %= MOD;
    }
    return (numerator * denomenator) % MOD;
}

ll bin_search(ll key, ll A[], ll left, ll right){
    // return the index idx where A[idx] = key.
    // A[left] is start and A[right] is end..
    // In other words, A[right], not A[right - 1], must be defined.
    while(right >= left){
        ll mid = left + (right - left) / 2;
        if (A[mid] == key) return mid;
        else if (A[mid] > key) right = mid - 1;
        else if (A[mid] < key) left = mid + 1;
    }
    return -1;
}

ll lower_bound_bin_search(ll key, ll A[], ll left, ll right){
    if (A[left] > key) return 0;

    while(right >= left){
        ll mid = left + (right - left) / 2;
        if (A[mid - 1] < key && key <= A[mid]) return mid;
        else if (A[mid] > key) right = mid - 1;
        else if (A[mid] < key) left = mid + 1;
    }

    return -1;
}

ll upper_bound_bin_search(ll key, ll A[], ll left, ll right){
    if (A[right] < key) return right;

    while(right >= left){
        ll mid = left + (right - left) / 2;
        if (A[mid] <= key && key < A[mid + 1]) return mid;
        else if (A[mid] > key) right = mid - 1;
        else if (A[mid] < key) left = mid + 1;
    }

    return -1;
}

ll fact(ll n){
    if (n == 0) return 1;
    return n * fact(n - 1);
}

bool is_prime(ll n){
    if (n <= 1) return false;
    for(ll i = 2; i * i <= n; i++){
        if (n % i == 0) return false;
    }
    return true;
}

ll bool_sum(ll a1, ll a2){
    if (a1 == 1 || a2 == 1) return 1;
    return 0;
}

mll prime_decomposition(ll n){
    ll i = 2;
    mll table;
    while(i * i <= n){
        while(n % i == 0){
            table[i] ++;
            n /= i;
        }
        i++;
    }
    if (n > 1) table[n] = 1;
    return table;
}

ll char_to_idx(char c){
    ll idx = 0;
    Forchar(cc, 'a', 'z'){
        if (c == cc) return idx;
        else idx ++;
    }
}

/*
    # 1. The usage of map pair
    map<pl, ll> cnt;
    cnt[{i, j}] = 0;
    items(kv, cnt){
        prl2(kv.first, kv.second);
    }

    # 2. The usage of pair sort

    vpl c;
    rep(i, n){
        ll a, b;
        in2(a, b);
        c.pb(mp(a, b));
    }
    sort(all(c));

    # 3. The usage of next_permutation and combination (factorial search)
    ll a[8];
    rep(i, 8) a[i] = i;
    do{
    }while(next_permutation(a, a+n));

    // here, combination
    ll n, r;
    ll i1, i2, ..., ir;
    For(i1, n - r, n) For(i2, n - r - 1, i1) ... For(ir, n - 2 * r + 1, irr){ process;}

    # 4. sort vector in descending order
    sort(rall(v));

    # 5. exponential order search
    ll n;
    in1(n);
    const ll base = 3;
    ll upper = 1;
    rep(i, n) upper *= base;
    rep(i, upper){
        ll tmp = i;
        rep(j, n){
            rep(k, base) if (tmp % base == k) prl(k);
            tmp /= base;
        }
    }

    # 6. imos method
    // used when we would like to count the number which
    // shows how many times the numbers between l and r belongs to smt.
    // This method is composed of three process.
    ll n, m, s[MAX_M], l, r;
    in2(n, m);
    rep(i, m) s[i] = 0;

    // 1st step
    rep(i, n){
        in2(l, r);
        s[l - 1]++; s[r]--; // if l starts from 1.
        // s[l]++; s[r + 1]--; // if l starts from 0.
    }

    // 2nd step
    rep(i, m - 1) s[i + 1] += s[i];
    // 3rd step: judgement...

    #7. shakutori method
    // 1. strech right side while the condition is met.
    // 2. renew the answer
    // 3. increments left side
    // 4. Back to 1. (l <= r must be satisfied all the time.)

    ll n, s[MAX_N]
    in1(n);
    rin1(i, n, s);

    while (l < n){
        r = max(r, l);
        if (l == r){val = s[l]; r++;}
        while(r < n && val * s[r] <= k){val *= s[r]; r++;}
        if (val <= k) answer = max(r - l, answer);
        val /= s[l]; l++;
    }

*/

const ll MAX_N = 200005;
ll n, k, p[MAX_N];
ll l[MAX_N], r[MAX_N];

int main(void){
    in2(n, k);
    rin1(i, n, p);
    l[0] = 0;
    r[0] = 0;

    rep(i, n){
        l[i + 1] = l[i] + p[i];
        r[i + 1] = r[i] + p[n - i - 1];
    }

    ll answer, idx, cnt;
    answer = 0;

    rep(i, n - k + 1){
        cnt = l[n] - l[i] - r[n - i - k];
        if (answer <= cnt) {answer = cnt; idx = i;}
    }

    ld e = 0;
    For(i, idx, idx + k){
        e += ld(p[i] + 1) / ld(2);
        // 1/p + 2/p + ... + p/p = (p + 1) / 2
    }

    prl_nd(e, 9);

    return 0;
}


Measured memory_bytes: 4992