Найти число целочисленных решений уравнения с ограничениями

(defun task ()
  (let ((lst nil))
    (dotimes (x1 10 t)
      (dotimes (x2 7 t)
        (dotimes (x3 12 t)
          (when (= 20 (+ x1 (+ x2 1) (+ x3 2))) (push (list x1 (+ 1 x2) (+ 2 x3)) lst)))))
    (printline lst)
    (length lst)))     
 
==> task
(task)
 
((9 7 4) (9 6 5) (9 5 6) (9 4 7) (9 3 8) (9 2 9) (9 1 10) (8 7 5) (8 6 6) (8 5 7)
 (8 4 8) (8 3 9) (8 2 10) (8 1 11) (7 7 6) (7 6 7) (7 5 8) (7 4 9) (7 3 10) (7 2 11) 
 (7 1 12) (6 7 7) (6 6 8) (6 5 9) (6 4 10) (6 3 11) (6 2 12) (6 1 13) (5 7 8) (5 6 9)
 (5 5 10) (5 4 11) (5 3 12) (5 2 13) (4 7 9) (4 6 10) (4 5 11) (4 4 12) (4 3 13)
 (3 7 10) (3 6 11) (3 5 12) (3 4 13) (2 7 11) (2 6 12) (2 5 13) (1 7 12) (1 6 13) (0 7 13))
 
==> 49

(defun task ()
  (let ((lst nil))
    (dotimes (x1 10 t)
      (dotimes (x2 7 t)
        (dotimes (x3 14 t)
          (when (= 20 (+ x1 (+ x2 1) (+ x3 2))) (push (list x1 (+ 1 x2) (+ 2 x3)) lst)))))
    (printline lst)
    (length lst)))
 
==> task
(task)
((9 7 4) (9 6 5) (9 5 6) (9 4 7) (9 3 8) (9 2 9) (9 1 10) (8 7 5) (8 6 6) (8 5 7) (8 4 8) (8 3 9) (8 2 10) (8 1 11) (7 7 6) (7 6 7) (7 5 8) (7 4 9) (7 3 10) (7 2 11) (7 1 12) (6 7 7) (6 6 8) (6 5 9) (6 4 10) (6 3 11) (6 2 12) (6 1 13) (5 7 8) (5 6 9) (5 5 10) (5 4 11) (5 3 12) (5 2 13) (5 1 14) (4 7 9) (4 6 10) (4 5 11) (4 4 12) (4 3 13) (4 2 14) (4 1 15) (3 7 10) (3 6 11) (3 5 12) (3 4 13) (3 3 14) (3 2 15) (2 7 11) (2 6 12) (2 5 13) (2 4 14) (2 3 15) (1 7 12) (1 6 13) (1 5 14) (1 4 15) (0 7 13) (0 6 14) (0 5 15))
 
==> 60