Ggmap: how to highlight city limits?

Hi there!
I would like to highlight the city limits (of Berlin, see example below), because you hardly recognise them. Do you have any ideas how to do this? Dashed, thicker lines in grey/black would be great.

library(ggplot2)
library(ggmap) 

myLocation<-c(lon= 13.405057, lat = 52.528357)
myMap<-get_map(location=myLocation, source="google", maptype= "roadmap", crop=FALSE, language = "German", color="bw") 
ggmap(myMap)

Thank you very very much in advance!!

Could you please turn this into a self-contained reprex (short for reproducible example)? It will help us help you if we can be sure we're all working with/looking at the same stuff (and it takes care of rendering images for you, so we'll be able to see the maps!).

install.reprex("reprex")

If you've never heard of a reprex before, you might want to start by reading the tidyverse.org help page. The reprex dos and don'ts are also useful.

What to do if you run into clipboard problems

If you run into problems with access to your clipboard, you can specify an outfile for the reprex, and then copy and paste the contents into the forum.

reprex::reprex(input = "fruits_stringdist.R", outfile = "fruits_stringdist.md")

For pointers specific to the community site, check out the reprex FAQ, linked to below.