editing permalinks

I have a blog that just moved from blogger and I am trying to make my permalinks match the old blogger address so that “pins” on pintrest will be still active. The permalinks are identical except for the fact that the old blogger URL’s have “.html” at the end and the WordPress links simply end with “/” as shown below:

Blogger: https://www.nightowlcorner.com/2013/04/40-birthday-party-themes-for-girls.html
Wordpress: https://www.nightowlcorner.com/2013/04/40-birthday-party-themes-for-girls-html/

When I try to edit the permalink in WordPress by adding the “.html” it reverts to “-html” when I save it, as shown here: https://www.nightowlcorner.com/2013/04/40-birthday-party-themes-for-girls-html/

How can I prevent the dot from becoming a dash?