Please help me to solve this css style issue.

I have a multiple div tags, with class of 'a' or 'b'.
And I want to differentiate css style for every 2nd div tag with class 'a', without care of 'b'.

```css
.a:nth-of-type(2n) {
  background-color: chocolate;
}
```

```html
<div class="parent">
  <div class="a">class a</div>
  <div class="a">class a</div>
  <div class="b">class b</div>
  <div class="a">class a</div>
  <div class="a">class a</div>
  <div class="b">class b</div>
  <div class="a">class a</div>
  <div class="b">class b</div>
  <div class="b">class b</div>
</div>
```

But it does not work, and my CSS always works without care of .a or .b.
This is execution result.
[![screen capture](https://i.stack.imgur.com/40WVX.png)](https://i.stack.imgur.com/40WVX.png)