def overlap(a1, a2, b1, b2):
    """
    Check if ranges a1-a2 and b1-b2 overlap.
    """
    a_min = min(a1, a2)
    a_max = max(a1, a2)
    b_min = min(b1, b2)
    b_max = max(b1, b2)
    
    return a_min <= b_min <= a_max or b_min <= a_min <= b_max or \
           a_min <= b_max <= a_max or b_min <= a_max <= b_max