Angular.js:如何为价格添加用户输入并保持小计

<div ng-app="myApp" ng-controller="myController">
  <label for="price">价格：</label>
  <input type="number" id="price" ng-model="price" ng-change="calculateTotal()" required>
  
  <label for="quantity">数量：</label>
  <input type="number" id="quantity" ng-model="quantity" ng-change="calculateTotal()" required>
  
  <label for="subtotal">小计：</label>
  <input type="number" id="subtotal" ng-model="subtotal" readonly>
</div>

angular.module('myApp', [])
  .controller('myController', function($scope) {
    $scope.price = 0;
    $scope.quantity = 0;
    $scope.subtotal = 0;
    
    $scope.calculateTotal = function() {
      $scope.subtotal = $scope.price * $scope.quantity;
    };
  });