Web Service in ASP.NET (Return JSON)

Hi mate. I saw your articles and i have an question with that issue. I have to make a calculator with web service asp.net with format of JSON. Simply there should be some request  

POST /add

Takes two real numbers, a and b, returns sum as res.

METHOD: POST

{

"a": 1

"b": 3

}

STATUS: 200

{

"res": 4

}

how can i do it with asp.net
Could you help me pls?