Six students, P, Q, R, S, T and U have been instructed to keep their books on six different shelves of a bookshelf having only six shelves one over the other. Books of S are kept on the topmost shelf. Books of P and U are kept on third shelves from the extreme top and extreme bottom, respectively. Only books of R are kept between the shelves containing books of U and Q. Books of student T are kept on which shelf from the extreme top?