Calculate average withouot incorporating sub average

Hi, for my work I have to calculate the average of cases for a year and the data is set up with monthly average in the row. I also need to not incorporate the cells with 0 or a dash in them because they are referenced to another worksheet with formula. The data is set up this way:

dates Cases
1/08 1
1/15 3
1/25 4
1/30 2
1st period: =IF(SUM(B2:B5)>0,AVERAGE(B2:B5),0)
(average)
. ((the next 11 mos. are repeated)
.
.
12/1 -
12/8 -
12/16 -
12/23 -
12/30 -
12th prd: =IF(SUM(B76:B79)>0,AVERAGE(B76:79),0)
(average)

Average: (Average up to the date)


At the end, I want to calculate the average "up to date", but I'm having problem selecting range with just the subaverage cells because Excel won't let me. Can you please help me calculating the average up to date?

Thank you!