I'm pretty new to angular (as you might guess). I'm trying to understand the best way to change content depending on what gets input in fields. 

For example, I have 

```
<div ng-app = "">

    <p>Enter variable 1: <input type = "number" ng-model = "var1"></p>

    <p>Enter variable 2: <input type ="number" ng-model ="var2"></p>

```

And I'm trying to display the product of the two variables, once both have been entered: 

```
    <p>Your total value is
    {{ (var1 * var2).isFinite? (var1 * var2) :  'Please enter a number' }} </p>

```

However, this doesn't work - it always displays 'Please enter a number'.

What is the best way to do this?