[R] R-help? how to take difference in next two elements

[dott]

Hi,

I'd like to take difference for a sequence a between a_i and a_i-2, for
instance,
a<-c(2,3,4,8,1)
I need (2, 5, -3) as a result. If not using a for loop, can anyone help me?
Thanks a lot.

Dot
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