Dropdown menu without invisible button?

I’m trying to get a simple drop down menu (horizontal nav) to close when the mouse rolls over to the next main nav button on its right. I don’t want the two parellel buttons to have any space for the invisible button .
The problem I am facing is that the menu doesn’t close when the user moves the mouse quickly to the next main button.

I hope I could get this across clear enough. Please help with a work around to this.

Thanks in advance.