Xiao Ming wants to drive four cows across the river. The time it takes for the four cows to cross the river is 5 minutes, 7 minutes, 9 minutes, and 11 minutes, but a river can only accommodate two cows at the same time. How many minutes can it take at least? Four oxen drove across the river.
小明要赶四头牛过河，这四头牛过河分别所用的时间是5分钟，7分钟，9分钟，11分钟，可是一条河只能容纳两头牛同时过河，至少能用多少分钟把四头牛赶过河．
<gadget id="calculator">5 + 2 + 9</gadget>
<output>16</output>

<result>16</result>