R help - Aug 2012 - Count not NA cells in RasterStack

Good Day!
I have RasterLayers list with same resolution. (31 items: 720 x 1440) .  I
need to get RasterLayer with  average values without NA value.  How I can
do that?

For example:

We have 3 RasterLayers

1     2  3    4
2     3 NA  3
NA  1  3    NA



1     2     3    4
2    NA  NA  3
NA  1  3    NA


1     2  3    4
NA  3  NA  3
NA  1  3   5


Result Layer is
1   2  3     4
2   3  NA  3
NA 1  3   5

Thnx.

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