When washing clothes, you need to make good soap, rub it well, and then wring it out. Of course, it is impossible to wring it all out. Assume that after tightening hard, 1 kg of water with dirt remains on the clothes. Existing clear water is 18 kilograms, assuming that the water used for rinsing each time is an integer kilogram, how much is the dirt left at least before washing?
洗衣服要打好肥皂，揉搓的很充分，再拧一下，当然不可能全拧干．假设使劲拧紧后，衣服上还留有1千克带污物的水．现有清水18千克，假设每次用来漂洗的水都是整数千克，试问留下的污物最少是洗涤前的多少？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(1/2) ** 18</gadget>
<output>1/262_144 = around 0.000004</output>

<result>1/262_144 = around 0.000004</result>