The following diagram shows a Zener diode as a voltage regulator. The Zener diode is rated at $V_{\mathrm{z}}=5 \mathrm{~V}$ and the desired current in load is 5 mA . The unregulated voltage source can supply upto 25 V. Considering the Zener diode can withstand four times of the load current, the value of resistor $R_s$ (shown in circuit) should be $\_\_\_\_$ $\Omega$.