def not_safe_gerone(a, b, c):
    """
    Compute triangle area from valid sides lengths

    :param float a: first side length
    :param float b: second side length
    :param float c: third side length
    :return: float -- triangle area

    >>> not_safe_gerone(3, 4, 5)
    6.0
    """
    p = (a+b+c)/2
    return (p*(p-a)*(p-b)*(p-c))**(1/2)