out.println(answer);
- output with maximal possible accuracy - WA24
out.printf("%.4f", answer);
- output with exactly 4 digits after decimal point - AC.

By the way, AC source 2369154 (3 Dec 2008) used println. Since then only Java 5->Java 6 changed. And maybe checker.

So is this behaviour bug of println or checker?

Read http://acm.timus.ru/forum/thread.aspx?id=22534&upd=633770517053517500

This situation is common to all Timus problems with float checkers. It will be fixed next week. All submits with wrong verdicts will be rejudged. If you want to get AC before rejudge, use output with a fixed number of decimal digits without exponential notation.