There are 50 girls who have fair or dark skin and blue or brown eyes. If 14 have blue eyes and fair skin, 31 have dark skin, and 18 have brown eyes, then brown How many dark-skinned girls are there?
有50个女孩，她们的皮肤有白的或浅黑色的，眼睛则是蓝色的或褐色的，如果有14个蓝眼睛白肤色，31个是浅黑肤色，18个是褐色眼睛，那么褐色眼睛浅黑肤色的女孩有多少个？
<gadget id="calculator">50 - 18 - 14</gadget>
<output>18</output>

<gadget id="calculator">31 - 18</gadget>
<output>13</output>

<result>13</result>