AndroidStudio doesn't recognize R

Question

I am teaching android app programming at a highschool. I found Eclipse to fragile so we are now trying out Android Studio. But a couple of students get that Android Studio with R. in red font, showing that it is not recognized.

This happened for example when the student copied the contents of my files.

Answer 1

It sometimes happens when you open a proyect from other sources. Just Synchronize it or press Ctrl-Alt-Y.