Display posts from top carent category only

Hello everyone! Sorry me for my bad English. I need help with some code I can’t get to work. I want to display all the posts from a parent category only on my index page. But the problem is, if the post is associated with a child category I don’t want to show it.

For example, if I have a post called “Test post” and the following categories: General, -Subgeneral 1, -Subgeneral 2

…and the post is filled under General, display it.

But if the post is filled under General and Subgeneral 1 or 2, don’t display it.

Thanks in advance! =)