[R] How can I draw a line in empirical distribution function?

[kiotoqq]

I've got the graphic with this:
a=c(120,40,75,85,55,75,55,90,90,55,155)
plot(ecdf(a))

How can I draw lines to show the lower quatile and upper quartil?

I only know there is the argument lines(),

but I don't know how to use it
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