title: Add two pandas dataframes with different indices
body: <p>I have two dataframes, each one having a lot of columns and rows. The elements in each row are the same, but their indexing is different. I want to add the elements of one of the columns of the two dataframes.</p>
  
  <p>As a basic example consider the following two Series:</p>
  
  <pre><code>Sr1 = pd.Series([1,2,3,4], index = [0, 1, 2, 3])
  Sr2 = pd.Series([3,4,-3,6], index = [1, 2, 3, 4])
  </code></pre>
  
  <p>Say that each row contains the same element, only in different indexing. I want to add the two columns and get in the end a new column that contains <code>[4,6,0,10]</code>. Instead, due to the indices, I get <code>[nan, 5, 7, 1]</code>.</p>
  
  <p>Is there an easy way to solve this without changing the indices?</p>
  
  <p>I want output as a series.</p>