Needed: Calculation formule with different columns

Dear everyone,

First of all thanks alot for helping!
I have already alot of data in my excel form, now i am creating a new colom in which I want to calculate some data from other colums.

Let me draw a small sketchs as example:
A B C
1 1 848 (All data which belong to the category A=1 (all numbers from B who belong to an 1 @ A)
2 1 343
3 1 234
4 2 232
5 2 45
6 2 567
7 2 565
8 3 920
9 3 842
10 3 34

My question is, what is the formula for C in order to save me alot of time in doing it by hand?
Sorry for my bad explenation, and my even worse english. I hope some1 can help me, thanks alot!!

Kind regards, Elmer