def divide(numerator, denominator):
    """Only allow exact division.

    Args:
        numerator (int): Numerator of the division.
        denominator (int): Denominator of the division.

    Returns:
        int: the result of exact division.
    """
    assert denominator != 0, 'denominator can not be zero'
    assert numerator % denominator == 0, \
        '{} is not divisible by {}'.format(numerator, denominator)
    return numerator // denominator