The smallest 6-digit number is 100,000 .
The largest 6-digit number is 999,999 .
If you count from the smallest one to the largest one, you'll name
every possible one.
But that's exactly the same thing as counting from 1 to 999,999 except
skipping the first 99,999 because they don't have enough digits.
So you're left with (999,999 minus 99,999) = 900,000 numbers remaining
