c# - Getting all files from a certain directory except a certain extension

Question

I would like to get all the files from a directory except the files that have a certain extension.

In my directory I have the following files:

file1.txt
file1.ok
file2.txt
file2.ok
file3.txt
file3.ok 
file4.txt
file5.xml
file6.ok

I can get the "ok"-files by using Directory.GetFiles(sourceDirectory, "*.ok") But how can i get a list of all the other files? A list of all the files that do not have the extions ".ok".