How do I automate chapter numbers in Pages?

I'm writing a book, which uses a Heading style for the heading of each chapter.

I want to number each chapter in words, opposed to numbers.

For example:

CHAPTER ONE

---

CHAPTER TWO

---

And so on.

Is there an easy way to do this as currently, if I insert or delete chapters while editing, I have to then manually update all subsequent chapter number headings?

Thanks in advance.