rope is 50 meters long, 21 meters are used for the first time, and 12 meters are used for the second time. How much is the length of the rope shorter than before? what is the correct formula
一根绳子长50米，第一次用去21米，第二次用去12米，现在绳子的长度比原来短了多少米？正确的算式是多少
<gadget id="calculator">21 + 12</gadget>
<output>33</output>

<result>33</result>