R reference in Android Studio

I find that the "R" reference encouters error in my Android Studio. Any idea ?

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Chris Harte2015 Top Expert (Most Article Points)Commented:
Clean and rebuild your project. R.java is a production file written by the compiler as it builds your project so cannot be edited. If the problems persist try shutting down Studio, deleting R.java, then opening Studio.  

When Studio first came out I had some R errors that would not go away. I ended up deleting the entire project and starting again, having saved all my source code first. I have not had anything like that for years and there should not be anything that is not cured by a Clean and Build.

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