Cannot resolve symbol R. How to solve?

Question

Android

Problems in such lines of code by default: setContentView(R.layout.activity_main)
in the file MainActivity.java
I did not touch this file at all in this project, added a couple of buttons, a TextViewer and changed the Background of the main Layout.
Clean Project and sync with Graddle files didn't help.

Answer 1

Take a look at the imports, there may be a wrong import of the R class. R also does not find when the project was unsuccessfully built.

Answer 2

file | Project Structure
dominoc925.blogspot.ru/2014/11/fixing-android-stud...