Expression Builder

 Hi Experts,

I have got a requirement where a value for a field has to be calculated based on the value selected in the picklist.

Lets say A , B and C are fields and X is a picklist with values say M,Nand O.

Now the requirement is that if 

1. C=M then A=B

2. C=N then A=B-N

3. C=O then A=B+O

I am new to OD, and am not able to figure out how to do this.

Any help is welcomed.

Thanks & Regards 

Sablok