Physics help?!!!!!!!!!!!!!!!!!!!!!!!?

There are four identical conducting spheres that are actually well separated from one another. Sphere W (with an initial charge of zero) is touched to sphere A and then they are separated. Next, sphere W is touched to sphere B (with an initial charge of qB = -27e) and then they are separated. Finally, sphere W is touched to sphere C (with an initial charge of qC = +45e) and then they are separated. The final charge on sphere W is +18e. What was the initial charge on sphere A?