There are 50 students running or skipping rope on the playground, including 18 girls, 31 students skipping rope, and 14 boys running. How many girls are there who skip rope?
操场上有50名同学在跑步或跳绳，其中女生有18名，跳绳的同学有31名，跑步的男生有14名．跳绳的女生有多少名？
<gadget id="calculator">50 - 18 - 14</gadget>
<output>18</output>

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

<result>13</result>