Octopus explicit "goal"

Hello, I am trying to create a solver where genomes are number of apartment and random seed of apartment types(studio, 1b2p, etc.) Both of these vary the overall length of the apartment run, which I would like to keep at a firm 35m, but I would also like to maximise people housed (sum(sum of each apt type x inhabitants)).
I have read that I need to turn the ‘number housed’ objective negative in order for it to be maximised, but how can I tell octopus that I am looking specifically for the sum of the length of all these apts. to be 35m?