title: Trying to build project after adding values-21 folder and it's showing error
body: <p>:app:mergeDebugResources FAILED
  C:\Users\abc\AndroidStudioProjects\MyApplication2\app\src\main\res\values-21
  Error:Error: Invalid resource directory name
  Error:Execution failed for task ':app:mergeDebugResources'.</p>
  
  <blockquote>
    <p>C:\Users\abc\AndroidStudioProjects\MyApplication2\app\src\main\res\values-21: Error: Invalid resource directory name</p>
  </blockquote>