{
 "results": [
   {
    "id": 1,
    "category": "Venta",
    "created_at": "2019-05-10",
    "total": 100
  },
  {
    "id": 2,
    "category": "Venta",
    "created_at": "2019-04-10",
    "total": 200
  }
 ]
}

JSONArray mJsonArray = response.getJSONArray("results");
  for (int i = 0; i < mJsonArray.length(); i++) {
   JSONObject mJsonObject = mJsonArray.getJSONObject(i);
   totalMont += mJsonObject.getInt("total");