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Common Lisp has multiple binding forms:

defparameter — top-level dynamic binding:

(defparameter *pi* 3.14159)    ; convention: earmuffs for globals

defvar — like defparameter but only sets if undefined.

let — local lexical binding:

(let ((x 10)
      (y 20))
  (+ x y))    ; 30

let* — sequential let (later bindings can refer to earlier):

(let* ((x 10)
       (y (* x 2)))
  (+ x y))    ; 30

Reading stdin:

(let ((line (read-line)))
  (parse-integer line))

read-line reads one line. parse-integer parses to int. Easier shortcut for numeric input: (read) which auto-parses Lisp atoms.