I mean, the imaginary friends took their decisions and got their consequences. The (real) person is ok. I guess it was just a dramatic end to their √(-1) friends

I know somebody that had. Their imaginary friends died of a drug overdose… so, yeah…

“they” may be aliens or robots

They are called “arrows”. Those, in particular, are red

making the BOLD ASSUMPTION that the angle of the arch is 90deg (the bottom right corner of your diagram), then the dashed lines will lead you to the value of the bold line.

If the original assumption is correct, then the answer is 15.

Ctl-D is the End-of-File character. Programs interpret it as “that’s it, the input you were reading has finished”, and react accordingly.