Took me about 2 - 3 days to answer.
Clue: [spoiler]Think about ways to represent a number rather than a sequence[/spoiler]
If you want give me any number and I'll solve
Nice idea but 15 would then be 7 not 8
The clue is look how numbers can be represented....
Letters is a nice idea
What other ways
BIG CLUE:
[spoiler]think coding[/spoiler]