What is the front camera's deviceUniqueID?

I am trying to write an app that involves both front and rear camera and switching between them. As far as I understand, in the addVideoInput method, I have to change the IDs in

    AVCaptureDevice *videoDevice = [AVCaptureDevice deviceWithUniqueID:(NSString *)deviceUniqueID];

But which NSStrings are those IDs?

Or, if it should be done in the other way, please, give a suggestion.

Thank you for help!