Nevermind. Solved.

I've detected my error with this input:

11 10
1 2
1 3
2 4
2 5
3 6
3 7
4 8
5 9
6 10
7 11

5 roads can be blocked.

Edited by author 29.08.2011 22:33

Thx!

The answer is:
5
1 2
4 8
5 9
3 6
7 11

Thank you very much! I tested your case and detected an error too. After fixing a bug, I got an AC. And that's mostly because of you!