EXCEL: finding largest gap within 9 rows

I have this excel spreadsheet and I need to calculate the biggest gap between the numbers below. (The gap cannot exceed 9 rows)

Example:
8/31/07 - 47.32
9/7/07 - 52.41
9/14/07 - 50.63
9/21/07 - 48.72
9/28/07 - 54
10/5/07 - 51
10/12/07 - 48
10/19/07 - 55
10/26/07 - 58
11/2/07 - 85
11/9/07 - 116
11/16/07 - 127
11/23/07 - 200
11/30/07 - 169
12/7/07 - 156
12/14/07 - 143
12/21/07 - 152
12/28/07 - 160
Biggest gap: 152

I cannot manually do every single row (this excel is gigantic). Isn’t there some excel formula to do this for me automatically?

I tried: =MAX(B1:B18) – MIN(B1:B18). Here I get the biggest gap between B1 (47.32) and B13 (200), but this gap is bigger then 1 month = 9 rows, which it is not allowed to exceed.

Thanks for your help.