Formula for calculating sequence number in third column based upon first 2 columns

Dear Friends,

I am facing an issue where I want to form a sequence in third column based upon the values in first 2 columns. More detailed description is given below:

The numbering in column C should restart from 1 as soon as the value in column A changes. In the below example the value is changing from 100 to 101 and resetiing the value in C again to 1 as soon as there is a change in column A. Column B are subset of column A for which the sequencing in column C is required.
A B C
100 12 1
100 15 2
100 17 3
100 19 4
101 6 1
101 8 2
101 9 3
101 10 4

Many thanks for your help in advance.
Arun