Android : r.class error android studio, I've tried all

I've a problem with R.class, android studio tells me: "Cannot resolve symbol R". I've tried all: -clean -rebuild -make But nothing...

In another post here in stackoverflow I read to uncheck use external builds in compiler in settings, but with the new version 0.8.9 this option disappear. Someone can tell me what I have to do? Thanks in advance :D

Sorry for my bad english >.<