My question is not particularly about this problem,
but about printf output with precision 10^-6 or better.

In this problem when I use float and output with printf("%.6f", ... ) I get WA8.
When I use double and printf("%.6lf", ... ) I get WA1.
What is the correct approach please?

I later got AC with this algorithm, it was correct as I had expected.
What I did was use double for the points' coordinates and output with ".6f" as if the were float.
Why does that work and my other version doesn't?