The contribution of the Aix-Marseille-Provence metropolis mentioned in Article L. 2513-5 is equal, pro rata to the number of inhabitants served, to the difference between the contribution called in 2015 by the Bouches-du-Rhône departmental fire and rescue service, in respect of the communes that made up the Marseille Provence Métropole urban community on 31 December of the same year, and the total contributions due from these communes in the year prior to their integration into the urban community.