(defun main (&rest argv)
  (declare (ignorable argv))
  (let* ((a (read))
         (b (read))
         (c (read))
         (d (read))
         (ans 0))
    (loop for i from a to b do
          (loop for j from c to d
                when (/= i j)
                  do (incf ans)))
    (format t "~d~%" ans)))

(main)