def check_digit_10(isbn):
    assert len(isbn) == 9
    sum = 0
    for i in range(len(isbn)):
        c = int(isbn[i])
        w = i + 1
        sum += w * c
    r = sum % 11
    if r == 10: return 'X'
    else: return str(r)

def check_digit_13(isbn):
    assert len(isbn) == 12
    sum = 0
    for i in range(len(isbn)):
        c = int(isbn[i])
        if i % 2: w = 3
        else: w = 1
        sum += w * c
    r = 10 - (sum % 10)
    if r == 10: return '0'
    else: return str(r)

def validate(isbn):
    if len(isbn) == 10:
        return check_digit_10(isbn[:-1]) == isbn[-1]
    elif len(isbn) == 13:
        return check_digit_13(isbn[:-1]) == isbn[-1]
    else:
        return False

def convert_to_10(isbn):
    if not validate(isbn):
        return
    if len(isbn) == 10:
        return isbn
    assert len(isbn) == 13
    prefix = isbn[3:-1]
    check = check_digit_10(prefix)
    return prefix + check

def convert_to_13(isbn):
    if not validate(isbn):
        return
    if len(isbn) == 13:
        return isbn
    assert len(isbn) == 10
    prefix = '978' + isbn[:-1]
    check = check_digit_13(prefix)
    return prefix + check