Help on explaining Cowntact Tracing

This is the link of the problems: Link here

I have read it so many time but still, I don’t understand it. I tried to downloaded test cases and run it by hand but still not right. I think I thought in wrong way so if anyone could help me explain this problems, I would appreciate it <3 <3.

You can examine a test case and explain it to me.

Is there a specific part you don’t get, or is it just the entire problem?

The entire problem

Why doesn’t the sample case make sense? It seems to be pretty clear.