Your task is to calculate `ab` mod `1337` where `a` is a positive integer and `b` is an extremely large positive integer given in the form of an array.

Example 1:

    Input: a = 2, b = [3]
    Output: 8

Example 2:

    Input: a = 2, b = [1,0]
    Output: 1024

Example 3:

    Input: a = 1, b = [4,3,3,8,5,2]
    Output: 1

Constraints:

- `1 <= a <= 231 - 1`
- `1 <= b.length <= 2000`
- `0 <= b[i] <= 9`
- `b` does not contain leading zeros.