int getIsbnChecksum( string );// C++

using namespace std;

int getIsbnChecksum( string );// Prototype for this function.

int getIsbnChecksum( string isbnBase ) {
    int sum = 0;
    for ( int i = 0; i < 12; i++ ) {
        if ( i % 3 != 0 )
            sum += int( isbnBase[ i ] - '0' );
        else
            sum += int( isbnBase[ i ] - '0' ) * 3;
    }
    int checksum = ( 10 - sum % 10 ) % 10;

    return checksum;
}