title: AngularJS, how to make two input field's sum always equals to a third input whenever they dynamically change?
body: <p>Here is a difficulty to me, there are three input fields, type number:</p>
  
  <p>field 1____ (*required), field 2____, field 3____</p>
  
  <p>how to use angularjs to achieve these:
  I. if no input on field 2 and field 3, leave them blank;
  II. whenever enter any of them, the other's value will be automatically updated so that their sum should always be the value in field 1;
  III. they have validations like: cannot be greater than the value in field 1, or negative.</p>