markdown - How to quickly fill characters of a specific length in vim?

Question

When using vim to write rst (reStructuredText) documents, I encountered such a demand.
In rst syntax, the next line of the title needs to be filled with ====.

this is h1
==========

I want to quickly fill in this paragraph ====, and the length is the length of the title of the previous line.

Answer 1

yypVr=