title: Pandas rolling gives NaN
body: <p>I'm looking at the tutorials on window functions, but I don't quite understand why the following code produces NaNs.</p>
  
  <p>If I understand correctly, the code creates a rolling window of size 2. Why do the first, fourth, and fifth rows have NaN? At first, I thought it's because adding NaN with another number would produce NaN, but then I'm not sure why the second row wouldn't be NaN.</p>
  
  <pre><code>dft = pd.DataFrame({'B': [0, 1, 2, np.nan, 4]}, 
                     index=pd.date_range('20130101 09:00:00', periods=5, freq='s'))
  
  
  In [58]: dft.rolling(2).sum()
  Out[58]: 
                         B
  2013-01-01 09:00:00  NaN
  2013-01-01 09:00:01  1.0
  2013-01-01 09:00:02  3.0
  2013-01-01 09:00:03  NaN
  2013-01-01 09:00:04  NaN
  </code></pre>