FRACTRAN, 0 bytes
To quote the Wikipedia article, a FRACTRAN program is run by updating an input integer n
using the following rules:
For the first fraction
f
in the list for whichnf
is an integer, replacen
bynf
Repeat this rule until no fraction in the list produces an integer when multiplied by
n
, then halt.
Rule 2 applies immediately because, well, there's no list.
(Note: I'm assuming here that input/output refers to the input and output integers, which is the closest alternative.)