Today, there are four people a, b, c, and d who will go from the left side of the bridge to the right side at night. Only two people can walk on this bridge at a time, and there is only one flashlight. You must use a flashlight when crossing the bridge. The fastest time for four people to cross the bridge is as follows: a: 2 minutes; b: 3 minutes; c: 8 minutes; d: 10 minutes. People who walk fast have to wait for people who walk slowly, so how many minutes can everyone cross the bridge at the fastest time?
今有a、b、c、d四人在晚上都要从桥的左边到右边．此桥一次最多只能走两人，而且只有一支手电筒，过桥是一定要用手电筒．四人过桥最快所需时间如下为：a：2分；b：3分；c：8分；d：10分．走的快的人要等走的慢的人，那么最快能在多少分钟内让所有的人都过桥．
<gadget id="calculator">3 + 2 + 10 + 3 + 3</gadget>
<output>21</output>

<result>21</result>