Count Column A only up to last nonblank cell in Column B

I need to count the number of days in column A with the condition of last nonblank cell in Column B. In the ex. below the answer should return 18.



Column A Column B
7/1/2011
7/2/2011 0.70612
7/3/2011
7/4/2011 0.6942
7/5/2011
7/6/2011 0.70612
7/7/2011
7/8/2011
7/9/2011
7/10/2011 0.70142
7/11/2011
7/12/2011
7/13/2011
7/14/2011
7/15/2011
7/16/2011
7/17/2011
7/18/2011 0.6956
7/19/2011
7/20/2011
7/21/2011
7/22/2011
7/23/2011
7/24/2011
7/25/2011
7/26/2011
7/27/2011
7/28/2011
7/29/2011
7/30/2011
7/31/2011