How many steps does it take to get from am to an (assume m < n)?
Suppose you have a staircase with n stairs, labeled from 1 to n. If you're standing on the mth stair,
there are (n – m) stairs left before you get to the top.
This translates nicely to sequences. If you're on the mth term of a sequence, and you want to get to the nth term, that means you have to take
n – m