The smaller the scope, the shorter the name. For 2 lines any reasonable (like l for "length") 1-letter name is OK.
l should be avoided in Python because PEP8 says

Never use the characters 'l' (lowercase letter el), 'O' (uppercase letter oh), or 'I' (uppercase letter eye) as single character variable names.

Although I'd just recommend using good fonts rather than forbidding letters.

I would go even further and avoid using one letter names for variables (maybe with a few exceptions). Here (CW) are few people who cares about readable code. I belive this is obvious enough to accept this.

Use spoiler flag next time, please. It's visible in the homepage if you don't.