Use the first X characters of a post as title?

I have a fairly large number of posts that are imported from other services and have no titles. My first idea to handle this was to use a custom tag and then show "untitled post" for these posts.

Unfortunately, it looks really silly on the archive page when there are several "untitled" posts in a row. My second idea to get the first X chars from the post and use that as the title, but is it possible to get the first X chars in some way?

And before anyone suggest using the name of the file, my posts are stored in folders with images and are all called "index.md" which means that I get an archive full of "index" which looks even more silly that "untitled post" 😁