(format t "Enter a: ") (force-output)
(defparameter a (read))
(format t "Enter b: ") (force-output)
(defparameter b (read))
(format t "Enter c: ") (force-output)
(defparameter c (read))
(format t "Enter y: ") (force-output)
(defparameter y (read))

(setf c (- c y))
(defparameter D (- (* b b) (* 4 a c)))

(cond ((< D 0)
       (format t "No real solution~%"))
      ((= D 0)
       (format t "One real solution: ~a~%" (/ (- b) (* 2 a))))
      (t
       (format t "Two real solutions: ~a and ~a~%"
               (/ (+ (- b) (sqrt D)) (* 2 a))
               (/ (- (- b) (sqrt D)) (* 2 a)))))