One morning in winter, it was snowing, and Xiao Ming set off from home to go to school. Due to the snow, Xiao Ming’s footprints in the snow would gradually disappear. 12 minutes after Xiao Ming set off, his father found that Xiao Ming had forgotten to bring his homework. After another 5 minutes, Dad began to find Xiao Ming’s footprints on the road; after another 7 minutes, Dad caught up with Xiao Ming, how many minutes did it take for Xiao Ming’s footprints to disappear from the moment they were stepped on?
冬季里的一天早上，天下着雪，小明从家出发去上学，由于下雪的缘故，小明留在雪地里的脚印会慢慢消失，小明出发12分钟后，爸爸发现小明忘记带作业本了，便去追小明，又过了5分钟，爸爸开始在路上发现小明留下的脚印；再过了7分钟，爸爸追上了小明，小明的脚印从刚踩下到消失需要多少分钟．
<gadget id="calculator">12 + 5 + 7</gadget>
<output>24</output>

<gadget id="calculator">5 + 7</gadget>
<output>12</output>

<gadget id="calculator">24 / 12</gadget>
<output>2</output>

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<gadget id="calculator">5 - 10</gadget>
<output>-5</output>

<gadget id="calculator">5 + 7 + (-5)</gadget>
<output>7</output>

<result>7</result>