How can I concatenate two sibling nodes in xslt?

My xml is like this

<root>
<level1>
    <value1>A</value1>
    <value2>B</value2>
    <value3>C</value3>
</level1>
<level1>
    <value1>D</value1>
    <value2>E</value2>
    <value3>F</value3>           
</level1>
</root>

I need a desired input as "A+D". How can i achieve this using XSLT version 2.0?