how to take cpInfoCurrentSlide system variable into user variable and subtract one to get the real number of the current slide.

In HTML5 there is actually a variable already.

window.cpInfoCurrentSlideIndex

Could you explain exactly why you want to do this? Because this variable is used often to insert the slide number on slides, it's index starts with 1 instead of the usual 0. Most other numbering system variables start with 0. That means if you want to replay a slide you also need an 'Expression' command like this:

Expression    cpCmndGotoSlide = cpInfoCurrentSlide - 1

Use either a Standard or Conditional Action and choose the Expression action. It will allow you to perform subtraction from a variable value.