Seven boxes O, P, Q, R, S, T and D are kept one over the other but not necessarily in the same order. Only three boxes are kept above T. Only one box is kept between O and T. Only three boxes are kept between O and D. D is kept at some place above T. P is kept immediately below D. S is kept at one of the places above R. Q is not kept immediately above or below O. How many boxes are kept between Q and R?