(defun read-points ()
  (list (read) (read) (read) (read) (read) (read)))

(defun calculate-area (x1 y1 x2 y2 x3 y3)
  (/ (abs (- (+ (* x1 (- y2 y3)) (* x2 (- y3 y1)) (* x3 (- y1 y2))))) 2.0))

(defun max-triangle-area (points)
  (let* ((x1 (nth 0 points))
         (y1 (nth 1 points))
         (x2 (nth 2 points))
         (y2 (nth 3 points))
         (x3 (nth 4 points))
         (y3 (nth 5 points))
         (deltas '((1 0) (-1 0) (0 1) (0 -1)))
         (max-area 0.0))
    (dolist (d1 deltas)
      (dolist (d2 deltas)
        (dolist (d3 deltas)
          (let* ((new-x1 (+ x1 (car d1)))
                 (new-y1 (+ y1 (cadr d1)))
                 (new-x2 (+ x2 (car d2)))
                 (new-y2 (+ y2 (cadr d2)))
                 (new-x3 (+ x3 (car d3)))
                 (new-y3 (+ y3 (cadr d3)))
                 (area (calculate-area new-x1 new-y1 new-x2 new-y2 new-x3 new-y3)))
            (setf max-area (max max-area area))))))
    max-area))

(let ((points (read-points)))
  (format t "~f~%" (max-triangle-area points)))