XSLT postive number to negative

How to check number (decimal or whole number ) in XSLT .I want to convert it to Negative if it is positive number otherwise i have to keep as it is . I verfied in google and all i see is negative number to positive number,using XSLT version as 1.0. Please give some samples.

Take below samples:

<Books>
 <Book>
  <Name>NC</Name>
  <Price>100.50</Price>
 </Book>
 </Books>


<Books>
 <Book>
  <Name>NC</Name>
  <Price>-200</Price>
 </Book>
 </Books>

Negative number to Positive:

<xsl:value-of select="Price * (Price >= 0) - Price * not(Price >= 0)" />

I want convert positive number to Negative and if number already Negative , i have to keep it as it is.

Regards, Chaitu