Setting values for variable

<xsl:variable name="pterm" select="0"/>
<xsl:for-each select="$lstAccount[Account_Type=$grpRecord]">
   <xsll:choose>
     <xsl:when test="string-length(normalize-space(Payment))!=0">
       <xsl:variable name="pterm" select="$pterm + Payment"/>
     </xsl:when>
   <xsl:otherwise>
      <xsl:variable name="pterm" select="$pterm + 0.02*Balance"/>
   </xsl:otherwise>
  </xsl:choose>
</xsl:for-each>
<xsl:value-of select="$pterm"/>

But this will generate error since pterm is declared more than once.
Anybody know what's the other way around this ?