restaurant has 27 signature dishes. Xiaoyue has eaten 13 of them, Dongdong has eaten 7 of them, and there are 2 dishes that both of them have eaten. May I ask: How many signature dishes are there that neither of you have tried?
某餐馆有27道招牌菜．小悦吃过其中的13道，冬冬吃过其中的7道，而且有2道菜是两人都吃过的．请问：有多少道招牌菜是两人都没有吃过的？
<gadget id="calculator">7 - 2</gadget>
<output>5</output>

<gadget id="calculator">13 + 5</gadget>
<output>18</output>

<gadget id="calculator">27 - 18</gadget>
<output>9</output>

<result>9</result>