(let* ((n (read))
       (k (read))
       (line (concatenate 'vector (loop repeat n collect (read))))
       (ans 0)
       (sum 0))
  (loop for i below n
        for j from (- 0 k)
        do
        (progn
          (incf sum (aref line i))
          (when (>= j 0)
            (decf sum (aref line j)))
          (setf ans (max ans sum))))
  (princ (/ (+ ans k) 2.0)))
