A brain teaser/math problem

[Kyon]

A mathematician, a biologist and a physics-expert are standing in front of a house. They see two people enter and three people leaving a while later. The biologists states: "This is a typical example of reproduction." The physics-expert says the measuring has some flaws, while the mathematician says: "No, no, you are wrong: When someone enters the building, the house is empty again."


2-3+1 = 0

Prove it wrong. :P