I'm trying to highlight select counties on a map of North Dekoda. How would I go about doing this? I have gotten this far so far. 

```
library(ggplot2)
library(maps)

county_df <- map_data('county')                          # mappings of counties by state
countyMap <- subset(county_df, region=="north dakota")   # subset just for NYS
countyMap$county <- countyMap$subregion

cnames <- aggregate(cbind(long, lat) ~ subregion, data=countyMap, FUN=mean)

ggplot(countyMap, aes(long, lat)) +  
  geom_polygon(aes(group=group), colour='black', fill=NA) +
  geom_text(data=cnames, aes(long, lat, label = subregion), size=3) +
  coord_map()

```

I would like to highlight the following counties.

```
countynames <- c("Adams",   "Billings", "Bottineau", "Bowman", "Burke",
                 "Divide", "Dunn", "Golden Valley", "Hettinger", "McHenry",
                 "McKenzie", "McLean", "Mercer", "Mountrail", "Renville", 
                 "Slope", "Stark", "Ward", "Williams")

```

Would this be possible?