title: Convert positive integer to negative integer
body: <pre><code>  def negate_amount
      amount = model.amount.to_s
      ("-" + amount).to_i
    end
  </code></pre>
  
  <p>is there a better way to turn positive integer to negative?</p>
  
  <p>The code above works, but is there a ruby or rails function for that? Without doing math operations?</p>