使用Xpath导航器获取xml文档中两个产品的平均价格

I have the following xml:

我有以下xml:

<products>
    <product id="1">
        <name>Cigarettes</name>
        <price>5</price>
        <stack>On stack</stack>
        <date>19.11.2016</date>
    </product>

    <product id="2">
        <name>Frozen vegetables</name>
        <price>10</price>
        <stack>Not on stack</stack>
        <date>22.08.2014</date>
    </product>
</products>
<product