The militia used 250 rounds of bullets for the first time, 320 rounds for the second time, and 180 rounds less than the sum of the previous two times for the third time. How many rounds were used for the third time?
民兵打靶，第一次用子弹250发，第二次用子弹320发，第三次比前两次的总和少180发，第三次用子弹多少发？
<gadget id="calculator">320 - 180</gadget>
<output>140</output>

<gadget id="calculator">250 + 140</gadget>
<output>390</output>

<result>390</result>