Li Jun rode on the back of the ox to drive the ox across the river. There are 4 cows in total, and the time required for these 4 cows to cross the river are: 1, 2, 5, 7 minutes respectively. Only two cows are driven across the river at a time, so how many minutes does it take for Li Jun to drive these four cows across the river?
李军骑在牛背上赶牛过河．共有4头，这4头牛过河所需时间分别为：1、2、5、7分钟．每次只赶两头牛过河，那么李军把这4头牛赶过河，至少需要多少分钟？
<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<gadget id="calculator">7 + 2</gadget>
<output>9</output>

<gadget id="calculator">3 + 9 + 2</gadget>
<output>14</output>

<result>14</result>