Xiao Zhang rides on the back of an ox to drive the ox across the river. There are four oxen A, B, C, and D in total. It takes 1 minute for A ox to cross the river, 2 minutes for B ox to cross the river, 5 minutes for C ox to cross the river, and 5 minutes for D ox to cross the river. It takes 6 minutes to cross the river. At most two oxen are driven across the river each time, and Xiao Zhang rides on the back of the oxen to cross the river each time. How many minutes will it take at least to drive all 4 cows to the opposite bank?
小张骑在牛背上赶牛过河，共有A、B、C、D四头牛，A牛过河需1分钟，B牛过河需2分钟，C牛过河需5分钟，D牛过河需6分钟．每次最多赶两头牛过河，而且小张每次骑在牛背上过河．要把4头牛都赶到对岸去，最少需要几分钟？
<gadget id="calculator">2 + 1 + 6 + 2 + 2</gadget>
<output>13</output>

<result>13</result>