Xiao Ming rides on the back of an ox to drive the ox across the river. There are four oxen A, B, C, and D in total. It takes 1 minute for A ox to cross the river, 2 minutes for B ox, 5 minutes for C ox, and 6 minutes for D ox. You can only ride one cow at a time. Cow, drive a cow across the river, please help Xiao Ming think about how many minutes it takes at least to cross the river.
小明骑在牛背上赶牛过河，共有甲乙丙丁四头牛，甲牛过河需1分钟，乙牛需2分钟，丙牛需5分钟，丁牛需6分钟，每次只能骑一头牛，赶一头牛过河，请你帮小明想一想，过河时间最少需多少分钟．
<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<gadget id="calculator">6 + 2</gadget>
<output>8</output>

<gadget id="calculator">3 + 8 + 2</gadget>
<output>13</output>

<result>13</result>