Subtraction by decimal number - xslt-2.0

Question

I have a to subtract the amount -1 based on the condition.Please any one help.
Input:
<JD>
<GP xmlns="">
I xmlns="">
<PK>40</PK>
<A/>
<AMNT>11659650.15</AMNT>
<B/>
<C/>
</I>
<I xmlns="">
<PK>50</PK>
<A/>
<AMNT>11659650.15</AMNT>
<B/>
<C/>
</I>
</GP>
</JD>
Tried with below XSLT and got 1.165964915E7 for 50.
<xsl:for-each select="JD/mo:GP/I">
<xsl:if test="PK='40'">
<xsl:variable name="a" select="AMNT"/>
<xsl:element name="AMT">
<xsl:value-of select="$a"/>
</xsl:element>
</xsl:if>
<xsl:if test="PK='50'">
<xsl:variable name="a" select="AMNT"/>
<xsl:element name="AMT">
<xsl:value-of select="$a - 1"/>
</xsl:element>
</xsl:if>

Answer 1

Considering your given input as following:
<?xml version="1.0" encoding="UTF-8"?>
<Root>
<JD>
<GP>
<I>
<PK>40</PK>
<A />
<AMNT>11659650.15</AMNT>
<B />
<C />
</I>
<I>
<PK>50</PK>
<A />
<AMNT>11659650.15</AMNT>
<B />
<C />
</I>
</GP>
</JD>
</Root>
In XSLT 2.0, you can try it using xs:decimal as below:
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:xs="http://www.w3.org/2001/XMLSchema"
exclude-result-prefixes="xs">
<xsl:output omit-xml-declaration="yes" indent="yes" />
<xsl:strip-space elements="*" />
<xsl:template match="/Root">
<xsl:for-each select="JD/mo:GP/I">
<xsl:if test="PK='40'">
<xsl:variable name="a" select="AMNT" />
<xsl:element name="AMT">
<xsl:value-of select="$a" />
</xsl:element>
</xsl:if>
<xsl:if test="PK='50'">
<xsl:variable name="a" select="AMNT" />
<xsl:element name="AMT">
<xsl:value-of select="xs:decimal($a) - 1" />
</xsl:element>
</xsl:if>
</xsl:for-each>
</xsl:template>
In XSLT 1.0, Use the format-number() function:
<xsl:template match="/Root">
<xsl:for-each select="JD/mo:GP/I">
<xsl:if test="PK='40'">
<xsl:variable name="a" select="AMNT" />
<xsl:element name="AMT">
<xsl:value-of select="$a" />
</xsl:element>
</xsl:if>
<xsl:if test="PK='50'">
<xsl:variable name="a" select="AMNT" />
<xsl:element name="AMT">
<xsl:value-of select="format-number($a - 1, '0.##')" />
</xsl:element>
</xsl:if>
</xsl:for-each>
</xsl:template>