XSLT: how to increment number value if attribute present

i have the following input

<xml>
      <Flight ID="F1">
      <Flight ID="F2">
      <Flight ID="F3" AddNew="Yes">
      <Flight ID="F4">
</xml>
      
      

I want my output to read:

<xml>
       <Flight ID="1"/>
      <Flight ID="2"/>
      <Flight ID="4"/>
      <Flight ID="5"/>
</xml>      



I have following xsl .which loops through each @ID and builds the ID by grabbing number after 'F'. And this is ok but I want to increment the number if AddNew="Yes" exists.  



<xsl:variable name="Seg" select="//Flight/@ID"/>


<xsl:for-each select="$Seg">

<xsl:variable name="ID">
  <xsl:value-of select="substring-after(current(), 'F')"/>
</xsl:variable>


<xsl:attribute name="ID" >
<xsl:value-of select="$ID"/>
</xsl:attribute>

.....