A barrel of oil weighs 6 kilograms. After half of the oil is poured out, the barrel still weighs 4 kilograms. How many kilograms does the oil weigh?
一桶油连桶重6千克，倒出一半油后，连桶还重4千克，油重多少千克．
<gadget id="calculator">6 - 4</gadget>
<output>2</output>

<gadget id="calculator">2 * 2</gadget>
<output>4</output>

<result>4</result>