import re
from datetime import datetime


def fromisoweek(week_string):
    """Calculates the datetime from the specified ISO Week string.

    Args:
        week_string: The ISO week string.

    Returns:
        The datetime object.
    """
    pattern_isoweek = r'^\d{4}-W\d{2}$'

    if re.search(pattern_isoweek, week_string):
        year = int(week_string[:4])
        week = int(week_string[-2:])
        return datetime.fromisocalendar(year, week, 1).astimezone()

    return None