def isbn13to10(isbn13: str) -> str:
    """
    Convert ISBN-13 to ISBN-10 (without checking validity of input).
    """
    isbn9 = isbn13[3:12]
    # for the checksum, isbn9 is zipped with [10, 9, ... 2]
    weights = range(10, 1, -1)
    checksum: int = sum(weight * int(digit) for weight, digit in zip(weights, isbn9))
    checkdigit: int = 11 - (checksum % 11)
    checkdigit_str: str = "X" if checkdigit == 10 else str(checkdigit)
    return isbn9 + checkdigit_str