Force on one half due to the other

Electricity and Magnetism Level 4

Two half-spheres with fixed surface charge densities $\sigma_1$ and $\sigma_2$ and radius $R$ are brought into contact, as shown to the right.

Find the force of interaction between the two half-spheres.

\frac{\sigma_1 \sigma_2}{8\epsilon_0} \pi R^2

\frac{\sigma_1 \sigma_2}{6\epsilon_0} \pi R^2

\frac{\sigma_1 \sigma_2}{4\epsilon_0} \pi R^2

\frac{\sigma_1 \sigma_2}{2\epsilon_0} \pi R^2