Need a formula.

Column 1 (type) Column 2 (wk)
a 77
b 77
c 78
d 80
e 80
blank 81

I would like the formula to come back for every week and tell me the number
of types that are in that week. Another words in this case there are qty 2
types in wk 77. In week 81 there are 0. In wk 78 there is 1 and week 80
there is 2.

Can anybody help?

Thanks.