def growth(born_day):
    """
    Calculate the growth of the fish population given one fish.

    Args:
        born_day (int): The day the initial fish was born.

    Returns:
        int: The number of fish that is a descendant of the initial fish.
    """
    count = 1
    born_day -= 2
    while True:
        born_day -= 7
        if born_day <= 0:
            return count
        else:
            count += growth(born_day)