Need to pick the value which is not starting with particular letter

I'm having the input with combination of alphabets and numbers, So i just want to pick the value of first which shouldn't start with 'B'

input xml:

<Group>
    <map>
        <Num>B123456</Num>
    </map>
    <map>
        <Num>555555</Num>
    </map>
    <map>
        <Num>666666</Num>
    </map>
    <map>
        <Num>B65432</Num>
    </map>
</Group>

XSL I have tried:

<xsl:transform xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0">
   <xsl:output encoding="UTF-8" indent="yes" />
   <xsl:template match="Group">
       <groupNumber>
          <xsl:value-of select="map/Num[not(starts-with(., 'B'))][1]"/>
       </groupNumber>
   </xsl:template>
</xsl:transform>

Actual Output:

<groupNumber>555555 666666</groupNumber>

Expected output:

<groupNumber>555555</groupNumber>

I only need to get the first Num element value which is doesn't start with B

You can try the following XPath expression:

(map/Num[not(starts-with(., 'B'))])[1]