I moved to the 4th fl of a 4 story condo some months ago. When I moved here, I noticed dead flies in both bathrooms and both bedrooms, near the windows. Recently, the fly problem has gotten out of control and I have closed all ceiling air ducts and somehow, the damn flies are still finding their way in; though, a few of the ducts can't be opened/closed. It is driving me crazy. I do not understand how they are getting in when the ducts are closed in my master bath. Please help!

What kind of flies are they, common house fly or small black flies with a triangle shaped wing?