A shepherd boy rides a bull to cross the river. There are 4 cows, A, B, C, D, who need to cross the river. It takes 1 minute, 2 minutes, 5 minutes and 6 minutes for 4 cows to cross the river respectively. If the shepherd boy can only drive 2 cows across the river at a time, and he needs to ride a cow to return, how many minutes will it take to drive all the 4 cows to the opposite bank?
牧童骑牛赶牛过河，共有甲乙丙丁4头牛需要过河．4头牛过河分别需要1分钟，2分钟，5分钟，6分钟．如果牧童每次只能赶2头牛过河，且返回需骑牛，则将四头牛全部赶到对岸至少需要多少分钟．
<gadget id="calculator">2 + 1 + 6 + 2 + 2</gadget>
<output>13</output>

<result>13</result>