extracting centroids from map data

Hello all, I'm working on a map. I'd like to extract centroids of the counties below in order to plot them as points. How can I do this?

Many thanks, Andy

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library(ggplot2)
library(maps)
all_states <- map_data("county")
states <- subset(all_states, region %in% c("iowa",  "illinois", "indiana",
                                           "michigan", "minnesota","ohio",
                                           "wisconsin"))
p <- ggplot()
p <- p + geom_polygon( data=states, aes(x=long, y=lat, group = group),
                       color="grey10", fill="white" )
p <- p + theme(aspect.ratio=1)
p