The check digit" for a 10-digit ISBN is either a digit or the letter X (X denotes a check 135 digit value of 10). All the characters of a 13-digit ISBN are digits. The dashes, which sometimes appear in ISBNs, are not significant and will not be included in the input to your program. The check digit" for a 10-digit ISBN is either a digit or the letter X (X denotes a check 135 digit value of 10). All the characters of a 13-digit ISBN are digits. The dashes, which sometimes appear in ISBNs, are not significant and will not be included in the input to your program