A few days later, the security guard counts two people missing at the end of the day. He checks the conference room, but there is no meeting going on. He looks around the building for people and sees that there are two offices with their lights still on. Each office is only supposed to have one person in it, so the security guard knows that each office with its lights on has exactly one person working late.
The security guard doesn't want to bother the workers to make sure that they are in, so he counts each lit office as a person still in the building. The two lit offices account for the two missing people and the security guard is happy and goes home for the evening.
His new formula for people in the building is