Addition for a folder at WordPress default permalink structure

My website is hosted at root directory (public_html) for hosting so the URL format is as follows:

http://www.example.com

I want the homepage to be open as it is on main domain but to add a custom folder directory (directory name ‘wp’) to all my inner pages and posts. For example currently I have the page structure like this:

http://www.example.com/?p=123

But I want it to be open like this with ‘wp’:

http://www.example.com/wp/?p=123

What are the necessary steps needs to be taken for this?

Thanks in advanced!