Write a function, `CollatzCountdown`, that returns the number of steps necessary to reach 1 using the collatz countdown.
Write a function, `CollatzCountdown`, that returns the number of steps necessary to reach 1 using the [collatz countdown](https://en.wikipedia.org/wiki/Collatz_conjecture).
- It must return `-1` if `start` is equal to `0` or negative.
- It must return `-1` if `start` is equal to `0` or negative.