There are 50 students, the trousers they wear are white or black, and the tops are blue or red. If 14 students wear blue jackets and white trousers, 31 students wear black pants, and 18 students wear red jackets, how many students wear red jackets and black trousers?
有50个学生，他们穿的裤子是白色的或黑色的，上衣是蓝色的或红色的．若有14人穿的是蓝上衣白裤子，31人穿黑裤子，18人穿红上衣，那么穿红上衣黑裤子的学生有多少个．
<gadget id="calculator">50 - 14 - 31</gadget>
<output>5</output>

<gadget id="calculator">18 - 5</gadget>
<output>13</output>

<result>13</result>