Xiaopang rides on the back of an ox and leads 4 oxen across the river. It is known that the time required for the 4 oxen to cross the river is 4 minutes, 6 minutes, 3 minutes, and 9 minutes. Xiaopang can only drive two cows across the river at a time, and then ride another cow back. How many minutes will it take to bring four oxen to the opposite bank?
小胖骑在牛背上带4条牛过河，已知4条牛过河所需要的时间分别是4分钟、6分钟、3分钟、9分钟．小胖每次只能赶两头牛过河，再骑一头牛返回．要把四条牛带到对岸，最少要花多少分钟．
<gadget id="calculator">4 + 3</gadget>
<output>7</output>

<gadget id="calculator">9 + 4</gadget>
<output>13</output>

<gadget id="calculator">7 + 13 + 4</gadget>
<output>24</output>

<result>24</result>