@start:
subleq @a, @IN
subleq @b, @IN
subleq @count, @n_one
subleq @count, @b
@loop:
subleq @count, @one, @next
subleq @sum, @a
subleq @b, @b, @loop
@next:
subleq @b, @sum
subleq @OUT, @b
subleq @a, @a
subleq @b, @b
subleq @count, @count
subleq @sum, @sum, @start
; Variables
@a: .data 0
@b: .data 0
@count: .data 0
@sum: .data 0
; Constants
@one: .data 1
@n_one: .data -1

Seems like a lot of people got stuck on this one, so I posted a sample solution :)