Paste symbol and calculation in plot

I'm trying to annotate a density plot and I would like to have R calculate the
standard deviation and place it in the plot next to the standard deviation symbol
"sigma". I can successfully use the text command to paste "StDev =",round(sd(Data),digits=3))
on the plot. However, I have trouble when I want to replace "StDev" with the Greek
symbol sigma (See code below).

? ? ? ?set.seed(1)
? ? ? ?Data=rnorm(100)
? ? ? ?plot(density(Data))
? ? ? ?text(2,0.35,paste("StDev =",round(sd(Data),digits=3)))

Through searching the help files I figured out that I can paste the sigma symbol on the
plot using the command "expression".

? ? ? ?text(2,0.3,expression(paste(sigma," = 0.898")))

However, when I try to paste together the sigma symbol and the standard deviation
that R is calculating, the text of the command to calculate the standard deviation
shows up on the plot instead of the number which I am asking it to calculate.

? ? ? ?text(2.2,0.25,expression(paste(sigma," = ",round(sd(Data),digits=3))))

Can someone please point me to a resource that will help me figure this out?

Thank You,

Jonathan