Count backwards N cells
 

Hi all,
In col.B I want a formula to count back from C1 times, cells in column A but
to skip the blank and 0, and return row of first cell:

A B C
1: 1 1 2
2: 0 1
3: 1
4: 0 1
5: -2 1
6: 3 5
7: 5
8: 5
9: 1 6
10: -5 9
11: 5 10
12: 0 10
13: 10

B9: because A9 and A6 meet conditions, return row 6
C1 is variable.
I hope I did understand, any help would be appreciated.