# Series

Given a string of digits, output all the contiguous substrings of length `n` in
that string.

For example, the string "49142" has the following 3-digit series:

- 491
- 914
- 142

And the following 4-digit series:

- 4914
- 9142

And if you ask for a 6-digit series from a 5-digit string, you deserve
whatever you get.

Note that these series are only required to occupy *adjacent positions*
in the input; the digits need not be *numerically consecutive*.