How to access element of JSON using Qt

I have this Json object and I want to access the "duration" and show it on console using Qt :

{
 "kind": "youtube#videoListResponse",
 "etag": "\"cbz3lIQ2N25AfwNr-BdxUVxJ_QY/brZ0pmrmXldPPKpGPRM-8I4dDFQ\"",
 "pageInfo": {
  "totalResults": 1,
  "resultsPerPage": 1
 },
 "items": [
  {
   "kind": "youtube#video",
   "etag": "\"cbz3lIQ2N25AfwNr-BdxUVxJ_QY/PkTW6UN9MH0O2kDApjC3penIiKs\"",
   "id": "WkC18w6Ys7Y",
   "contentDetails": {
    "duration": "PT58M21S",
    "dimension": "2d",
    "definition": "hd",
    "caption": "false",
    "licensedContent": true,
    "projection": "rectangular"
   }
  }
 ]
}

And my Qt code is this :

{
    QJsonDocument jsonResponse = QJsonDocument::fromJson(message);
    results = jsonResponse.object();

    QJsonValue v1 = results.value("items");

    qDebug() << "v1 = " << v1;

    QJsonValue v2 = v1.toObject().value("contentDetails");

    qDebug() <<"v2 = " << v2;

    QString v3 = v2.toObject().value("duration").toString();

    qDebug() << "v3 = " << v3;
}

However my output is :

v1 = QJsonValue(array, QJsonArray([{"contentDetails":{"caption":"false","definition":"hd","dimension":"2d","duration":"PT58M21S","licensedContent":true,"projection":"rectangular"},"etag":"\"cbz3lIQ2N25AfwNr-BdxUVxJ_QY/PkTW6UN9MH0O2kDApjC3penIiKs\"","id":"WkC18w6Ys7Y","kind":"youtube#video"}]))

v2 = QJsonValue(undefined)

v3 = ""

So v1 is fine but v2 becomes undefined.What am I doing wrong and how can I access the "duration" item correctly?

The direct answer as follows:

// Read the file which has the JSON object.
QFile file("jsonString.json");
if(!file.open(QFile::ReadOnly)){
    qDebug()<< "Error, Cannot open the file.";
    return false;
}

QJsonDocument jsonDoc = QJsonDocument::fromJson(file.readAll());
qDebug()<< jsonDoc.object().value("items").toArray()[0].toObject().value("contentDetails").toObject().value("duration").toString();

The result: PT58M21S