(defun main (&rest argv)
  (declare (ignorable argv))
  (let* ((n (read))
         (a (make-array n :element-type 'integer))
         (dp (make-array (1+ n) :element-type 'integer :initial-element 0))
         (res n))
    (dotimes (i n) (setf (aref a i) (read)))
    (sort a #'<)
    (loop for i from n downto 0 do
          (when (and (< (+ i 1) n) (= (+ (aref a i) 2) (aref a (+ i 1)))) (setf (aref dp i) (1+ (aref dp (+ i 1)))))
          (when (and (< (+ i 2) n) (= (+ (aref a i) 2) (aref a (+ i 2)))) (setf (aref dp i) (1+ (aref dp (+ i 2))))))
    (loop for i below (1- n)
          when (/= (1+ (aref a i)) (aref a (1+ i)))
            do (incf res (1+ (aref dp (1+ i)))))
    (format t "~d~%" res)))

(main)