The distance between an object and a screen is 100 cm . A lens can produce real image of the object on the screen for two different positions between the screen and the object. The distance between these two positions is 40 cm . If the power of the lens is close to $\left(\frac{\mathrm{N}}{100}\right) \mathrm{D}$ where N is an integer, the value of N is $\_\_\_\_$ .