title: Get Sum from given String
body: <p>How can we get the result from a String?
  For example, if we are given following String</p>
  
  <pre><code>String value = "4 + 5 + - 3";
  </code></pre>
  
  <p>how can we get the result in int?
  For exapmple, the answer for the given String is "6".</p>