difference of two rows

Dear R user,
I'd like to calculate the difference of two rows, where "ID" is the same.
eg.: I've got the following dataframe:
ID YEAR
13 2007 
15 2003
15 2006
15 2008
21 2006
21 2007

and I'd like to get the difference, like this:
ID YEAR     diff
13 2007      NA
15 2003       3
15 2006       2
15 2008      NA
21 2006       1   
21 2007      NA  

that should be fairly easy...I hope
Thanks for any helpful comments 
B.

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