title: How to do substruction in the cells of columns in python
body: <blockquote>
    <blockquote>
      <p>I have this dataframe (df) in python:</p>
    </blockquote>
  </blockquote>
  
  <pre><code>  Cumulative sales
  0        12
  1        28
  2        56
  3        87
  </code></pre>
  
  <blockquote>
    <blockquote>
      <p>I want to create a new column in which I whould have the the number of new sales (N-(N-1)) as below:</p>
    </blockquote>
  </blockquote>
  
  <pre><code>  Cumulative sales   New Sales
  0        12             12
  1        28             16
  2        56             28
  3        87             31
  </code></pre>