Well, not sure about this formula but if you wanted to use one yourself, think about the steps you would take if you were doing the exchange of wrappers by hand; by the way, not the best code to write as it's confusing and you are writing over the money variable instead of creating a new one. First you buy as many chocolates as you want, chocolates = money/cost . Now you eat those chocolates and exchange all the wrappers possible for extra ones, let's call it extras = chocolates / extra .

BUT you could have leftover chocolates so let's see how many, remainders = chocolates % extra . Since we got extras , we can add it to the remainders and see if it is enough for more chocolates, super_extras = (extras + remainders) / extra . Remember that all of this is integer division so it'll round down.

Finish by adding to the total chocolates += extras + super_extras and you're done! You have to tweak it a bit to get edge cases but that part should be trivial. Use the same logic to see where those terms you wrote came from. I think if you rearrange what I showed you'll get the same thing.