How to compress only files without folders using the SharpZipLib library? (C#)

Question

I use the following code to create a zip file:

var files = Directory.GetFiles(targetPath);
var zip = ZipFile.Create(targetPath + "/myzip.zip");
zip.BeginUpdate();
foreach (var file in files) zip.Add(file, CompressionMethod.Deflated);
zip.CommitUpdate();
zip.Close();

I get the following structure of archive:

myzip.zip
   - Folder1
      - Folder2
        - File1.txt
        - File2.txt
        - File3.txt

But I need:

myzip.zip
  - File1.txt
  - File2.txt
  - File3.txt

Help please.