A Niu crosses the river on horseback. There are four horses A, B, C, and D. It takes 2 minutes for A to cross the river, 3 minutes for B to cross the river, 7 minutes for C to cross the river, and 7 minutes for Ding to cross the river. It takes 6 minutes, and only two horses can be driven across the river at a time. Ask: How long will it take at least to drive all 4 horses to the other side?
阿牛骑在马背上过河，共有甲、乙、丙、丁四匹马，甲马过河要2分钟，乙马过河要3分钟，丙马过河要7分钟，丁马过河要6分钟，每次只能赶两匹马过河，问：要把4匹马都未赶到对岸，最少要多少分钟？
<gadget id="calculator">3 + 2 + 7 + 3 + 3</gadget>
<output>18</output>

<result>18</result>