title: I want perform calculations "Add" by extension string. How can do it?
body: <p>I have 1 <code>EditText</code> and 4 buttons.</p>
  
  <pre><code>EditText: Show result
  Button1: Number 1
  Button2: Number 2
  Button3: Operation "+"
  Button3: Result "="
  </code></pre>
  
  <p>I want when press 1 + 1 + 2 + ... = Result.</p>
  
  <p>So How Can do this ?</p>