Https://www.codecademy.com/courses/learn-python/lessons/introduction-to-bitwise-operators/exercises/just-flip-out

Dear Colleagues,

https://www.codecademy.com/courses/learn-python/lessons/introduction-to-bitwise-operators/exercises/just-flip-out
I thought the mask was to be the exact opposite of a so l replace 1 with 0 and 0 with 1,so why is the mask with 1 all through

Thank you
Jagmeet