How is AIC calculated in stepAIC

Here is a very simple lm model from ?lm

ctl <- c(4.17,5.58,5.18,6.11,4.50,4.61,5.17,4.53,5.33,5.14)
trt <- c(4.81,4.17,4.41,3.59,5.87,3.83,6.03,4.89,4.32,4.69)
group <- gl(2,10,20, labels=c("Ctl","Trt"))
weight <- c(ctl, trt)
lm.D9 <- lm(weight ~ group)

If I use stepAIC to lm.D9, on the very first line it says AIC = -12.58

require(MASS)
stepAIC(lm.D9)

If I use AIC directly on lm.D9, it gives a different value 46.17648

AIC(lm.D9)

My question is why the 2 AIC values are different. Thanks!