Let be the surface with boundary obtained by deleting the interiors of the disks. By excision,
so and the other relative groups vanish. In the long exact sequence in relative homology, the map
sends an orientation class to , up to an overall choice of sign. It is therefore injective and has cokernel . Exactness gives and a short exact sequence
This sequence splits because its right-hand group is a free abelian group. Since is connected, we obtain