title: error in MainActivity.jave file
body: <p>i am editing mainactivity.java code file this error occur 
  <strong>R cannot be resolved to a variable</strong>
  when i import R then their further more new error occurs like 
  <strong>bAdd cannot be resolved or is not a field
  bSub cannot be resolved or is not a field
  tvDisplay cannot be resolved or is not a field</strong>
  kindly guide me how can i remove these errors </p>