I'm a F2P player

It's truly admirable that you're persevering without spending money, hmm! Zombie outbreaks and clearing Legend Stories are the basics for getting Cat Food. The most important thing is self-control, only using Cat Food during guaranteed draws, so don't forget that!

I don't recall anything specific... Besides getting the game and receiving canned food, your only option is daily login. If there's an event, make sure to clear the event map to the end and grab the rewards...

You do need some specs, but you can collect a lot by farming 2, 3, or 4-star Gurejendo or Shinrejendo. Good luck!

If you clear all stages, you get 30 cans, so you can clear event stages or Legend Story.