//
import java.math.*;
import java.util.*;
import java.io.*;
  
public class Main { 
  
    static BufferedReader in;
    static String file = "../in";
    static int test = 10; // 0 for local testing, 1 for std input
    static int inf = 1_000_000;

    public static void main(String[] args) throws Exception
    {
        int _k = Integer.valueOf("1");
        if(test > 0) in = new BufferedReader(new InputStreamReader(System.in));
        else in = new BufferedReader(new FileReader(file));
        if(test < 0) {String[] str = in.readLine().split(" ");}
/***********************************************************************/
/***********************************************************************/
/***********************************************************************/
/***********************************************************************/
        new Main().d();
    }

    static String[] split() throws Exception
    {
        return in.readLine().split(" ");
    }

    static int readInt() throws Exception
    {
        return Integer.valueOf(in.readLine());
    }
    

    void f() throws Exception
    {
        String[] sp = in.readLine().split(" ");
        int n = Integer.valueOf(sp[0]),
            d = Integer.valueOf(sp[1]),
            a = Integer.valueOf(sp[2]);

        int[][] ary = new int[n][2];

        int bound = 0;

        for(int i = 0; i < n; i++) {
            sp = in.readLine().split(" ");
            ary[i][0] = Integer.valueOf(sp[0]);
            ary[i][1] = Integer.valueOf(sp[1]);
            bound = Math.max(bound, ary[i][0]);
        }
        
        Arrays.sort(ary, (u, v)->(u[0] - v[0]));

        // SegmentTree root = new SegmentTree(0, ary[n - 1][0]);   //1_000_000_010);
        Queue<int[]> q = new PriorityQueue<>((u, v)->(u[0] - v[0]));
        long sum = 0;
        long re = 0;
        for(int i = 0; i < n; i++)
        {
            

            // for(int[] qq : q) System.out.println("current q.." + Arrays.toString(qq));
            
            // while(!q.isEmpty() && q.peek()[0] < ary[i][0]) q.poll();
            // long q = root.get(ary[i][0]);
            // System.out.printf("%d %d\n", ary[i][0], q);
            int maxd = ary[i][0] + 2 * d;
            while(!q.isEmpty() && q.peek()[0] <= ary[i][0] ) sum += q.poll()[1];
            // System.out.println("sum = .." + sum);
            ary[i][1] -= sum * a;

            // System.out.printf("current ary.. %s\n", Arrays.toString(ary[i]));
            if(ary[i][1] <= 0) continue;
            // for(int[] elm : ary)
            
            
            int op = ary[i][1] / a;
            if(ary[i][1] % a > 0) op++;
            re += op;
            q.offer(new int[]{ary[i][0], op});
            q.offer(new int[]{maxd + 1, -op});
        }

        System.out.println(re);
    }


    void e() throws Exception
    {
        String[] sp = in.readLine().split(" ");
        int h = Integer.valueOf(sp[0]),
            n = Integer.valueOf(sp[1]);

        int[] weight = new int[n];
        int[] cost = new int[n];

        int[] f = new int[20010];
        Arrays.fill(f, 1 << 30);
        f[0] = 0;
        for(int i = 0; i < n; i++)
        {
            sp = in.readLine().split(" ");
            int c = Integer.valueOf(sp[0]);
            int w = Integer.valueOf(sp[1]);
            for(int j = c; j <= 20000; j++)
            {
                f[j] = Math.min(f[j], f[j - c] + w);
            }
        }
        // System.out.println(Arrays.toString(f));
        for(int i = 20000; i >= 1; i--)
            f[i] = Math.min(f[i], f[i + 1]);

        // System.out.println(Arrays.toString(f));

        // for(int i = 1; i <= h; i++) 
        //     f[i] = Math.min(f[i], 1 + f[i - 1]);
        
        // System.out.println(Arrays.toString(f));
        System.out.println(f[h]);
    }

    long gcd(long a, long b)
    {
        if(b == 0) return a;
        return gcd(b, a % b);
    }

    void d() throws Exception
    {
        String[] sp = split();
        int n = Integer.valueOf(sp[0]),
            k = Integer.valueOf(sp[1]);
        sp = split();

        double[] db = new double[n];
        for(int i = 0; i < n; i++)
        {
            int v = Integer.valueOf(sp[i]);
            int sum = (1 + v) * v / 2;
            double avg = 1.0 * sum / v;
            db[i] = avg;
        }

        // every k window
        double maxi = 0.0;
        double runsum = 0.0;

        for(int i = 0; i < n; i++)
        {
            runsum += db[i];
            if(i >= k - 1)
            {
                if(i >= k) runsum -= db[i - k];
                maxi = Math.max(maxi, runsum);
            }
        }
        System.out.println(maxi);

    }
    
    boolean satisfy(int a, int b)
    {
        String s1 = String.valueOf(a),
                s2 = String.valueOf(b);
        int m = s1.length(), n = s2.length();
        if(s1.charAt(0) == s2.charAt(n - 1) && 
            s1.charAt(m - 1) == s2.charAt(0)) return true;
        return false;
    }

    void c() throws Exception
    {
        int n = readInt();
        String[] sp = split();
        Set<Integer> set = new HashSet<>();
        for(int i = 0; i < n; i++)
        {
            set.add(Integer.valueOf(sp[i]));
        }
        if(set.size() == n) System.out.println("YES");
        else System.out.println("NO");
    }

    void b() throws Exception
    {
        String str = in.readLine();
        char[] c = str.toCharArray();
        Arrays.fill(c, 'x');
        System.out.println(String.valueOf(c));
    }

    void a() throws Exception
    {
        String[] sp = split();
        String c1 = sp[0], c2 = sp[1];
        sp = split();
        int a = Integer.valueOf(sp[0]),
            b = Integer.valueOf(sp[1]);
        String u = in.readLine();
        if(c1.equals(u)) a--;
        if(c2.equals(u)) b--;
        System.out.printf("%d %d", a, b);
    }
}

Measured memory_bytes: 53332