How do you calculate the ROI ratio of a company if it is the first month of that company?

So for example, the company started on May 1st, and during that time, they had 27550 in total assets and 1500 in Net income.

If you're calculating the may ROI, would you need to still divide 27550 by 2 when getting the average total assets, or because its the first month and the company has a starting balance of 0 for assets, would you just use the 27550 as the average total assets.