def verify_isbn(isbn):
    
    if ( type(isbn) != str ):
        return False
    
    isbn = isbn.replace('-', '')
    
    if ( len(isbn) != 10 ):
        return False

    sum = 0
    count = 10
    for digit in isbn[0:-1]:
        sum += int(digit) * count
        count -= 1
      
    sum = (11 - ( sum % 11 )) % 11
    
    if ( sum == 10 ):
        return "X" == isbn[-1]
    else:
        return str(sum) == isbn[-1]