title: Simple arithmetic on values retreived using OIDs
body: <p>I am new to SNMP and Nagios, and would like to do some basic arithmetic on values retrieved using OIDs. </p>
  
  <p>I need to monitor the used memory (RAM) on the servers on the University's network. They seem to be using UCD-SNMP-MIB. So far I have encountered <code>memTotalReal.0</code> and <code>memAvailReal.0</code>, and no direct way to retrieve the used memory. Is there any specific reason for this?</p>
  
  <p>Expected command for basic subtraction:</p>
  
  <pre><code>$USER1$/check_snmp -H $HOSTADDRESS$ -o memTotalReal.0-memAvailReal.0
  </code></pre>
  
  <p>What is the easiest way to retrieve the used memory? Do I need to resort to subtraction by possibly writing another script?</p>