How to get Total unused User Licenses

String result;

String orgName;

String orgCountry;

         Organization orgDetails =[ select Id, Name, Country from Organization limit 1];

            Id orgId = orgDetails.Id;

            orgName = orgDetails.Name;

            orgCountry = orgDetails.Country;

        PageReference pr=new PageReference('/'+orgId);

        //called screenscraping: get the data from the page

        String rawData=pr.getContent().toString();

   String licRow = '>Salesforce</th><td class=" dataCell  ">Active</td><td class=" dataCell  numericalColumn">';

    Integer licLen = licRow.length();

  Integer pos=rawData.indexOf(licRow);

if (-1!=pos)

{

         Integer licStart = pos + licLen;

         result=rawData.substring(licStart, licStart+3);

}

Decimal lic = decimal.valueOf(result);

integer u = [select count() from user where profile.UserLicense.Name = 'salesforce' and isactive = true];

Decimal userCount = decimal.valueOf(u);

Integer totalUnused = (lic.intValue() - userCount.intValue());

System.debug('************ Licenses???? ***************** ' + String.valueOf(totalUnused));