ILL 1979 Problem 39

A desert expedition camps at the border of the desert, and has to provide one liter of drinking water for another member of the expedition, residing on the distance of $n$ days of walking from the camp, under the following conditions: $(i)$ Each member of the expedition can pick up at most $3$ liters of water. $(ii)$ Each member must drink one liter of water every day spent in the desert. $(iii)$ All the members must return to the camp. How much water do they need (at least) in order to do that?