How to create a report with full and part time EMP count as per department ?

I have created a headcount report with basic employee info. I want to modify it such that it has Emp ID, Dept ID, Full time & Part Time Empl count. Basically I am looking to add a column that will give full time empl count and part time empl count as per the department? I am using Assignment category field that has values like Full time - Regular, Full time - temporary , Part time - regular, Part time - Temporary. How do I edit the formula for this assignment category field such that it give full time and part