Permissions in SSRS

Hello,
I have the following scenario. Please go over it. Any advice will be helpful.
I have three folders(folder A,folder B,folder c) in the report manager and each folders have nearly 10 reports.
Now when i click folder A, click on properties and again on security, i see that group or user name is BUILTIN/Administrator and the role is Content manager. So, only builtin admins will have access to this folder and reports in the folder. Same is the case with all the other folders.

Now apart from builtin admins, i want to give access to another user(User A) only on one of those folders (folder A). When UserA is give access in Folder A, he should not be able to access folder B or folder C.

Thanks.