What is it that given one, you'll have either two or none

Sasha: My name is. I'm from Rossii.U us a nice apartment on the seventh floor of a multistory building. The apartment has three bedrooms. My favorite room -bedroom. It's small. The walls are yellow in it. In the bedroom - a sofa, an armchair, two chairs and a desk with a computer. Table by the window. Sofa and chair against the wall, they are orange. A big poster on the wall. This is my favorite movie-known hero Spider-Man. His name is Peter Parker. I have a lot of books. They are in the bookcase in the hall.