Generate and verify the check digit for the UPU S10 postal tracking codes.

All postal administrations in the world use 13-character tracking numbers, such as EB000717618HK.

The first two characters denote shipment type, the last two characters are the originating country.

The first eight digits are the shipment number, while the ninth digit (8 in this example) is the checksum based on the previous eight numbers.

You have the first eight digits and you want to generate the check digit.

const S10 = require('s10');

const digit = S10.calculateCheckDigit('00071761'); // 8

// OR:
// const digit = S10.calculateCheckDigit(71761); // 8

Returns true if the tracking number contain 13 characters (two letters, nine digits, two letters) and if the check digit is correct.

const S10 = require('s10');

const valid1 = S10.trackingNumberIsValid('EB000717618HK'); // true
const valid2 = S10.trackingNumberIsValid('EB900717618HK'); // false
const valid3 = S10.trackingNumberIsValid('1234567891011'); // false