Find the all "1" masking number which has the same "binary length" as the input number.

For example, input = 0d5, masking number = 0b111; input = 0d2, masking number = 0b11

```
def find_complement(num)
num ^ mask(num)
end
def mask(number)
(1 << number.to_s(2).length) - 1
end
```