Xiaojun's father earns 1,800 yuan a month, and his mother earns 1,600 yuan a month. Xiaojun's family needs 2,500 yuan for living expenses. How much can they have left over each month?
小军的爸爸每月收入1800元，妈妈每月收入1600元．小军一家的生活费需要2500元，每月能结余多少元？
<gadget id="calculator">1_600 - 2_500</gadget>
<output>-900</output>

<gadget id="calculator">1_800 + (-900)</gadget>
<output>900</output>

<result>900</result>