The problem statement says n >= 3. But there are testcases for n=1 and n=2 that assume the result to be -1. Sucks if you technically solved it during the contest but didn't get the full points (not me)

*In my opinion the testcases for n=1 and n=2 should be 0 at best and not -1. But on the other hand n=1 and n=2 make no sense since we don't have a 1- or 2-sided shape.