I'm styling my document and I need to number chapters using Roman numerals (I, II, III, …) but the sub-titles have to be all in decimal numbers, so it looks like this:

```
I. …
1.1. …
1.1.1. …
```

The problem is that Word uses the previous numerical notation for the next level, and it looks like this:

```
I. …
I.1. …
I.1.1. …
```

How can I change it?