using System;

using System.Linq;

using System.Collections.Generic;

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {       

        var json = 

        @"{

 ""kind"": ""youtube#videoListResponse"",

 ""etag"": ""\""NO6QTeg0-3ShswIeqLchQ_mzWJs/o3L50v5R2WwnOWFaoCXQIOzRQt8\"""",

 ""pageInfo"": {

  ""totalResults"": 1,

  ""resultsPerPage"": 1

 },

 ""items"": [

  {

   ""kind"": ""youtube#video"",

   ""etag"": ""\""NO6QTeg0-3ShswIeqLchQ_mzWJs/emOU-eLbb23xAEX_1JcXPAXYJUY\"""",

   ""id"": ""7eCPp1WLpyA"",

   ""contentDetails"": {

    ""duration"": ""PT7M10S"",

    ""dimension"": ""2d"",

    ""definition"": ""hd"",

    ""caption"": ""false"",

    ""licensedContent"": true

   },

   ""statistics"": {

    ""viewCount"": ""1603"",

    ""likeCount"": ""454"",

    ""dislikeCount"": ""6"",

    ""favoriteCount"": ""0"",

    ""commentCount"": ""107""

   }

  }

 ]

}";

        // using JObject.Parse

        var obj1 = JObject.Parse(json);

        var duration1 = obj1["items"][0]["contentDetails"]["duration"];

        Console.WriteLine(duration1);

        // using stongly typed deserialization

        var obj2 = JsonConvert.DeserializeObject<Response>(json);

        var duration2 = obj2.items.First().contentDetails.duration;

        Console.WriteLine(duration2);

    }

    public class Response

    {

        public IEnumerable<ResponseItem> items;

    }

    public class ResponseItem 

    {

        public ContentDetails contentDetails;

    }

    public class ContentDetails

    {

        public string duration;

    }

}