(b)(6%) Suppose that they have no access to the financial market, but can make the following arrangement. The father gives the son an amount of $R in the red envelope in 2021, and the son gives the father some pocket money $P in 2022, where R and P ≥ 0 (a negative amount means the opposite flow of money). Describe all the Pareto optimal pairs (R, P), if any, that both would agree, and illustrate in a figure. (There is no need to compute the exact number.)