Uncle Li rode on horseback to drive horses across the river. There are 5 horses in total. It takes 2 minutes, 3 minutes, 4 minutes, 6 minutes, and 7 minutes for these 5 horses to cross the river. Only 3 horses can be driven each time. How long will it take at least to drive all 5 horses to the opposite bank?
李大爷骑在马背上赶马过河，共5匹马，这5匹马过河分别要2分钟、3分钟、4分钟、6分钟、7分钟，每次只能赶3匹马，要把这5匹马都赶到对岸去，最少要多少分钟．
<gadget id="calculator">4 + 2 + 7</gadget>
<output>13</output>

<result>13</result>