Write a program to calculate the checksum$-$digit of a number. It is calculated as follows:

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Cont

Calculate the position of digits. Rightmost position starts at $0,$ Eg: in $1234,1$ is $3$rd digit, $4$ is Oth digit.

Double the digits at even positions only and subtract $9$ if the result is $>=10,$ so that it becomes a single digit. Eg: $8$ will become $7,$ as $8*2=16=>1+6=7.($which is also, $16-9)$

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Sum all the digits $($after previous step$)$ and if the result is a multiple of $10,$ checksum digit will be $0.$ Otherwise, checksum digit is the number required to make the sum a multiple of $10.$