allocate-registers a, b, one, factorial-product
   allocate-registers end, n, result, zero

   li one, 1
   li zero, 0
   li factorial-product, factorial-product-label
   read n
   li a, 1
   add b, zero, n ; copy n into b by adding zero
   li end, after-first ; note continuation is after-first

factorial-product-label:
   ;; computes a * b! into a and then jumps to end
   ;;  provided that b is a non-negative integer;
   ;;  assumes that the register named one contains 1 and
   ;;  the factorial-product register contains this address;
   ;;  may also change the b register's contents
   jeqz b, end   ; if b = 0, a * b! is already in a

   mul a, a, b   ; otherwise, we can put a * b into a
   sub b, b, one ; and b - 1 into b, and start the
   j factorial-product            ; iteration over

after-first:
   add result, zero, a    ; save n! away in result
   li a, 1
   add b, n, n            ; and set up to do (2n)!,
   li end, after-second   ; continuing differently after
   j factorial-product    ; this 2nd factorial-product,

after-second:             ; namely, by
   add result, result, a  ; adding (2n!) in with n!
   write result           ; and displaying the sum
   halt