After dinner, Xiao Ming and grandpa walked along the Hai River at an even speed, where many willow trees were planted at equal intervals. Now it is known that it took them 12 minutes to walk from the first tree to the 13th tree, and after a few more trees, they went back. When they walked back to the fifth tree, they shared the 50 minutes. You know, they started walking back when they reached the tree number.
吃完晚饭，小明和爷爷以均匀的速度在海河边散步，海河边间隔相等的种着很多柳树．现在知道从第1棵树走到第13棵树，他们用了12分钟，又往前走了几棵树后他们就往回走，当他们走回到第5棵树时，从出发起共用了50分钟．你知道，他们是散步到第多少棵树时开始往回走的．
<gadget id="calculator">13 - 1</gadget>
<output>12</output>

<gadget id="calculator">12 / 12</gadget>
<output>1</output>

<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">1 * 4</gadget>
<output>4</output>

<gadget id="calculator">50 + 4</gadget>
<output>54</output>

<gadget id="calculator">54 / 2 / 1</gadget>
<output>27</output>

<gadget id="calculator">27 + 1</gadget>
<output>28</output>

<result>28</result>