Sum values in excel

Hi all,

Lets say that I have these values in Column A:

5
20
7
12
18

I need to decrease the value of each cell by 1 and then: 1) sum these new values; 2) get an average of these new values.

Is it possible to write some smart function that would do that? For instance I wrote this formula:

=sum(A:A;-1)

(I thought that in this way excel will decrrease the cells values by 1 and then will sum new values), but it didn't work....

If you taking one of each value then new value is:

=SUM(A:A)-count(A:A)

and new average is

=-1+SUM(A:A)/count(A:A)

removed - over complex - zbor has resolved