Download files from google drive and save to a folder

Hi,
I need to download files from google drive and save to a folder on local using C#.
I have done as described in Google Developer API documentation (https://developers.google.com/drive/v3/web/manage-downloads) but i'm getting the file with invalid format. Please suggest how to download it.

I done:
downloadUrl = url of the file (eg: https://drive.google.com/uc?id=1ULNeKdDoCRmWgPPBW8-d1EGUZgqvA1Ul&export=download)
filename = some name with extension

private bool SaveToDir(string downloadUrl,string filename) {
string filePath = Server.MapPath("imports/");
bool resp = false;
DriveService ds = new DriveService();
Uri temp = new Uri(downloadUrl);
string fileId = HttpUtility.ParseQueryString(temp.Query).Get("id");
var req = ds.Files.Get(fileId.Trim());
var stream = new MemoryStream();
req.MediaDownloader.ProgressChanged += (Google.Apis.Download.IDownloadProgress dp)=> {
switch (dp.Status)
{
case Google.Apis.Download.DownloadStatus.Downloading:
Message("downloading, please wait....");
break;
case Google.Apis.Download.DownloadStatus.Completed:
using (FileStream file = new FileStream(filePath, FileMode.Create, FileAccess.Write))
{
stream.WriteTo(file);
Message("File Downloaded successfully.");
}
resp = true;
break;
case Google.Apis.Download.DownloadStatus.Failed:
Message("Failed to Download.");
resp = false;
break;
}
};
req.Download(stream);