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
the largest product for a series of 6 digits is 23520.
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