Handling input and output in automation of a functionality?

Let's say that you have to test a new way of getting a sum of two numbers from software and you have to test that functionality and do BDD automation. From below two what would be a better approach for automation (Also why)?

1) Using fixed input and expecting the same output. Ex: Input -> 3,5 Output -> 8

OR

2) Using random two number on every run and validating it against the conventional sum.