Number of combinations

Hey,
I need some help with the following issue...

Say I have N blocks of decreasing size (each block smaller than the previous),
and each block can either be put on another block of a bigger size, or alone.
How many combinations can I have? (without duplicates)

I can count that for 3 blocks I have 5:
[ 3 2 1 ]
[ 3 2 ][ 1 ]
[ 3 1 ][ 2 ]
[ 3 ][ 2 1 ]
[ 3 ][ 2 ][ 1 ]
But how do I calculate this for N?

Thanks!

Never mind, found it here:
http://en.wikipedia.org/wiki/Bell_number

Bell_numbers count the number of partitions of a finite set. Hence, they also count the number of equivalence relations of the set.