Complete reaction of acetaldehyde with excess formaldehyde, upon heating with conc. NaOH solution, gives P and Q . Compound P does not give Tollens' test, whereas Q on acidification gives positive Tollens' test. Treatment of P with excess cyclohexanone in the presence of catalytic amount of p -toluenesulfonic acid (PTSA) gives product R . Sum of the number of methylene groups (- CH ₂- ) and oxygen atoms in R is _____