While working for the police, you've identified a house where people go to commit crimes, called Crime House. One day, you set up a camera over the door of the house and record a video.
You don't know how many people were in Crime House at the start of the day, but you can see people enter and leave through the front door. Unfortunately, because the people entering and leaving Crime House are criminals, sometimes they wear masks; and you aren't quite sure if the front door is the only way in or out.
Sometimes you can guess who was wearing a mask. If criminal #5 entered the house, then someone wearing a mask left, then criminal #5 entered the house again, then either the person wearing the mask was criminal #5, or there is another way out of Crime House.
At the end of the day, when Crime House has closed its doors for the night, you watch your video. Because you're an optimist, you want to figure out if it's possible that there are no other entrances or exits from crime house; and if so, you want to figure out the minimum number of people who could be in Crime House at the end of the day.