The cowherd boy rides on the back of an ox to drive the ox across the river. There are 4 cows A, B, C, and D. It takes 2 minutes for A cow to cross the river, 3 minutes for B cow to cross the river, 6 minutes for C cow to cross the river, and 7 minutes for D cow to cross the river. Only two cows can be driven across the river at a time. Ask how many minutes it will take to drive 4 cows across the river.
放牛娃骑在牛背上赶牛过河．共有甲、乙、丙、丁4头牛，甲牛过河要2分钟，乙牛过河要3分钟，丙牛过河要6分钟，丁牛过河要7分钟．每次只能赶两头牛过河．问要把4头牛赶过河对岸，最少要花多少分钟．
<gadget id="calculator">3 + 2 + 7 + 3 + 3</gadget>
<output>18</output>

<result>18</result>