When Ding Ding was calculating a subtraction problem, he mistakenly regarded the subtrahend 31 as 13, and calculated the wrong number as 49. What should be the correct number?
小丁丁在计算一道减法题时把减数31错看成13，算出错误得数是49，正确的得数应该是多少．
<gadget id="calculator">31 - 13</gadget>
<output>18</output>

<gadget id="calculator">49 - 18</gadget>
<output>31</output>

<result>31</result>