Simple math in associative arrays?

Hi,

I have a problem with terms in associative arrays. Here is the relevant  
snippet of my template:


$data[$smarty.foreach.blocks.iteration 
-1][$smarty.foreach.items.iteration-1]


But this leads to a "unknown tag" error... How can I easily solve this?  
Maybe math does not work in array keys?


Manuel Holtgrewe