Field test
Every wrong answer costs a point, so guessing without care scores nothing — but skipping a problem you don’t know (leave it blank, press Enter) costs nothing either.
Your score is right minus wrong, so a blind guess is worth nothing on average.
The server times the run on its own clock and re-derives every problem to score it; a run that could not have been solved by hand is flagged and never ranks.
Type on your keyboard or the on-screen keypad — both work for every problem.