As far as I know, it's mandatory in Australia also to vote (strangely enough), where a non-voter might be fined. There are exceptions to the rule of course, like overseas residents etc.
If you are unhappy with either of the major parties, vote for a minor one, or if you don't want to vote at all, submit a "dummy" vote (not filling the voting slip out correctly).
Good luck in Brazil, hope it gets better