Could someone please help me on this problem?

A spring has a natural length of 20 cm. If a 25 N force is required to keep it stretched to a length of 30 cm, how much work is required to stretch it from 20 cm to 25 cm?

I know that work=(force)(distance)

Force=25 N

distance=25-20=5

work=(25N)(5cm)

Is this correct so far? If it isn't, could you please show me what to do?

Thank you very much