Xiao Wang rode an ox to drive an ox across the river. There are 4 cows A, B, C, and D. It takes 1 minute for A cow to cross the river, 2 minutes for B cow to cross the river, 5 minutes for C cow to cross the river, and 6 minutes for D cow to cross the river. Only two cows can be driven across the river at a time. How long will it take at least to drive all four cows to the other side of the river?
小王骑牛赶牛过河．共有甲、乙、丙、丁4头牛，甲牛过河需1分钟，乙牛过河需2分钟，丙牛过河需5分钟，丁牛过河需6分钟．每次只能赶两头牛过河．问要把这四头牛都赶到河对岸去，最少需多少分钟？
<gadget id="calculator">2 + 1 + 6 + 2 + 2</gadget>
<output>13</output>

<result>13</result>