The following checksum formula is widely used by banks and credit card companies to validate legal account numbers:
d0 + f(d1) + d2 + f(d3) + d4 + f(d5) + ... = 0 (mod 10)
The di are the decimal digits of the account number and f(d) is the sum of the decimal digits of 2d (for example, f(7) = 5 because 2 × 7 = 14 and 1 + 4 = 5). For example, 17,327 is valid because 1 + 5 + 3 + 4 + 7 = 20, which is a multiple of 10. Implement the function f and write a program to take a 10-digit integer as a command-line argument and print a valid 11-digit number with the given integer as its first 10 digits and the checksum as the last digit.
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