it is about python.

first picture is question. Second pictur is the answer post on chegg. I understand why we do 2^n. My question is why we need minus one

1.5 One bit can represent 0 or 1, with two bits you have four possible combinations 00, 01, 10, and 11. How many different combinations can you make with four bits or eight bits (one byte)? Each combination can be used to represent a binary number. How many numbers can you represent with 2 bytes (16 bits)? How many numbers can you represent with four bytes