title: Decrement or increment a variable in the robot framework
body: <p>I just want to decrement the variable N_groups in the last line.
  This is my robot file:</p>
  
  <pre><code>Preconditions - Delete Groups But Not First
      ${N_groups}    Setup Groups Count Groups
      Log to console    N_groups: ${N_groups}
      : FOR    ${INDEX}    IN RANGE    1    20
      \    Run Keyword If    '${N_groups}' == '1'    Exit For Loop
      \    Setup Groups Delete Group    ${group}
      \    ${N_groups}=    ${N_groups}-1
  </code></pre>
  
  <p>I get the error:</p>
  
  <p><code>No keyword with name '${N_groups}-1' found.</code></p>
  
  <p>What I am doing wrong here?</p>