FRACTRAN, 0 bytes

To quote the Wikipedia article, a FRACTRAN program is run by updating an input integer n using the following rules:

1. For the first fraction f in the list for which nf is an integer, replace n by nf

2. 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.)