Kaperkar’s Constant

I was recently introduced to Kaperkar’s Constant.
It is quite magical. You take any four digit number $A$ , sort the digits from highest to lowest to create a new number $A^{\text{high}}$ , sort the digits from lowest to highest to get $A^{\text{low}}$ , and calculate and new number $A= A^{\text{high}}- A^{\text{low}}$ . You repeat this procedure enough times and you end up with $A=6174$ .