ah... cockroaches are quite common in the US. Mostly because there are too many dirty people lol

Anyways, one of your neighbors could be lying food out which attracts them. They also don't like the cold.
Here's what you do, make sure you don't leave any kind of food unwrapped or out in the open because that will attract more. If you have an open box of any kind of food (pasta, cereal, ect) I suggest buying a few cheap containers. They cannot chew threw anything.

Also see if any of the stores have a spray for cockroaches what you bought it ok but the spray will prevent them from coming in, kill whats already in there, and it will also kill the eggs. When you get the spray, spray around all cracks that can come in from just keep a window open so you don't get sick from the fumes and keep it away from food and dinnerware. You can spray your sink too, just after you spray wait about ten minutes then run the hot water down the drain and clean the sink itself to avoid it getting on ur dishes. Put what you aleady have underneath your sink, one on the counter in a corner, and one near where you store food and maybe one near the garbage bucket.

That should take care of the problem. Give it a few days and if you still see some you may have to get a professional to come in.

Hope that helps and just remember cockroaches dont like clean houses. Clean as in no food anywhere not dust lol