Let's do some magic

You have a magic box that doubles the count of items you put, in every day. The given program takes the initial count of the items and the number of days as input. Task Write a program to calculate and output items' count on the last day. Sample Input 3 2 Sample Output 12 Explanation Day 1: 6 (3*2) Day 2: 12 (6*2) Solution: days = int(input()) items = int(input()) While days > 0: items = items*2 days = days - 1 print(items) Can someone explain why "days -= 1" in the code ? I don't understand.