; 2 点間のマンハッタン距離の半分を出力する
(defun main (&rest argv)
  (declare (ignorable argv))
  (let* ((px (read))
         (py (read))
         (qx (read))
         (qy (read))
         (ans (/ (+ (abs (- px qx)) (abs (- py qy))) 2)))
    (format t "~f~%" ans)))
(main)