(defun main (&rest argv)
  (declare (ignorable argv))
  (let* ((n (read))
         (xs (loop repeat n collect (read)))
         (ans 'T))
    (if (/= (abs (car xs)) 1)
        (setq ans 'F)
        (loop for x on xs
              while (< 1 (length x))
              as a = (car x)
              as b = (cadr x)
              when (/= (abs (- a b)) 1)
                do (setq ans 'F) (return)))
    (princ ans)
    (terpri)))
(main)