How do I select a value from json

Here json

{"ts":1827908701,"updates":[[4,30623409,17,81282347579,1425823449632," ... ","tfs"],[80,1,0],[7,81282347579,30652308]]}

How can I get the value 4 of updates using the library boost?

I know how to take such a value of "ts". But I do not understand how to take the value of the two brackets