Cook's distance

Dear useRs,

I have some trouble with the calculation of Cook's distance in R.
The formula for Cook's distance can be found for example here:
http://en.wikipedia.org/wiki/Cook%27s_distance

I tried to apply it in R:

y <- (1:400)^2
x <- 1:100
lm(y~x) -> linmod # just for the sake of a simple example
linmod$residuals[1]^2/(2*mean(linmod$residuals^2))*(hatvalues(linmod)[1]/(1-hatvalues(linmod)[1])^2)

1 
0.02503195

cooks.distance(linmod)[1]

1 
0.02490679 

Why differ the two results?

Thanks a lot if somebody have some instructions for me.

Best wishes:

Kolos