title: for build calculate and make errors
body: <p>in my project for build calculate has error like this :</p>
  
  <pre><code>void findView(){
  
          btn_0=(Button)findViewById(R.id.btn_0);
             btn_1=(Button)findViewById(R.id.btn_1);
  </code></pre>
  
  <p>and cannot resolve symbol for anythings have R ,what should i do ?</p>