def minstaff_rule(M, i, j, w):
    """
    Minimum staffing levels required

    :param M: Model
    :param i: period
    :param j: day
    :param w: week
    :return: Constraint rule
    """

    # Note no understaffing on lhs
    return M.cov[i, j, w] >= M.min_staff[i, j, w]