Will and Grant left school and walked eight tenths of a mile due south to Will's house to pick up his prize pig Petunia. Will, Grant and Petunia then walked four tenths of a mile directly east to Grant's house where his pooch named Pinocchio was waiting to join the party. The two boys, Petunia and Pinocchio then walked DIRECTLY back to school. How much shorter was their walk back to school than the walk to Grant's house?