Rolling Count Formula with Reset

I need some help constructing a formula to help do the follwing:

I have a set of data in Col F:
A
B 1
B 2
B 3
A 1
A 2
B 1
B 2
B 3
B 4
A 1


The numbers are what I am trying to accomplish in Row E. I am basically wanting to a conescutive occurence count.