Seven friends, M, N, O, P, Q, R and S live on seven different floors of the same building, having floors in such a manner that the ground floor is numbered 1, the floor above it number 2, and so on till the topmost floor is numbered 7. M lives on floor number 1. Q lives immediately above M. Only four friends live between the floors of Q and N. Only three friends live between the floors of P and M. R lives immediately below O. Who lives on the sixth floor?