plot two histograms together in R

Considering two samples of datas A and B. To plot histograms together in the same window 

just run the following commands (in the R Console) 

hist(A,col="red")
hist(B, col="blue",add=T)

Example: 

b=rnorm(2000, 6, 1)
a=rnorm(2000, 3, 1)

hist(a,col="red",ylim=c(0,400))

hist(b,col="blue",ylim=c(0,400),add=T)  # add=T is the key