Create Pie chart of frequency count with D3.js

I have a question regarding of d3.js

Let's assume that i have this data:

A A B K J I O N P B N J D G R X S O M H Y D J B T N Y S B N R N E B U D B G D G Y F T K O

i would like to get the frequency count of each letter and make a pie chart with D3.js