# Instructions

Given a string of digits, calculate the largest product for a contiguous
substring of digits of length n.

For example, for the input `'1027839564'`, the largest product for a
series of 3 digits is 270 (9 \* 5 \* 6), and the largest product for a
series of 5 digits is 7560 (7 \* 8 \* 3 \* 9 \* 5).

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

For the input `'73167176531330624919225119674426574742355349194934'`,
the largest product for a series of 6 digits is 23520.

For a series of zero digits, the largest product is 1 because 1 is the multiplicative identity.
(You don't need to know what a multiplicative identity is to solve this problem;
it just means that multiplying a number by 1 gives you the same number.)
