title: Sum all negative values of a df
body: <p>I Need to sum up all negative values in a df...</p>
  
  <p>I am trying to change this code to sum up negative values in a df. The desired outcome is to have a variable with the sum of all negative values in a df.</p>
  
  <pre><code> df.values.sum()
  </code></pre>
  
  <p>illustrative df code:</p>
  
  <pre><code>df = pd.DataFrame({'A' : ['foo', 'bar', 'foo', 'bar', 'foo', 'bar', 'foo', 'foo'],
                     'B' : ['one', 'one', 'two', 'three', 'two', 'two', 'one', 'three'],
                     'C' : np.random.randn(8), 'D' : np.random.randn(8)})
  </code></pre>