In the circuit shown in the figure, when key $K_1$ is closed, the ammeter reads $I_0$ whether $K_2$ is open or closed.

But when $K_1$ is open, the ammeter reads $\frac{I_0}{2}$, when $\frac{I_0}{2}$ when $K_2$ is closed.
Assuming that ammeter resistance is much less than $R_2$, the values of and $R_2$ in ohms are