title: Angular 5: Validate multiple input fields by sum up values
body: <p>I would like to validate multiple number input fields by sum up their values and create a custom Validator for Angular.</p>
  
  <p>Every Input looks like this:</p>
  
  <pre><code>&lt;input type="number" min="0" max="10"&gt;
  </code></pre>
  
  <p>There are multiple number inputs and everyone can have a value between 0 and 10, but the sum of all fields must be less or equal 10.</p>
  
  <p>I have a function that returns true if the sum of all input fields is less or equal 10. 
  But how I achieve this with a custom Angular Validator? So that the error message appear instantly.</p>