title: Sum values of similar keys inside two nested dictionary in python
body: <p>I have nested dictionary like this:</p>
  
  <pre><code>data = {
      "2010":{
              'A':2,
              'B':3,
              'C':5,
              'D':-18,
          },
      "2011":{
              'A':1,
              'B':2,
              'C':3,
              'D':1,
          },
      "2012":{
              'A':1,
              'B':2,
              'C':4,
              'D':2
          }
      }
  </code></pre>
  
  <p>In my case, i need to sum all values based on its similar keys in every year, from 2010 till 2012..
  So the result i expected should be like this:</p>
  
  <pre><code>data = {'A':4,'B':7, 'C':12, 'D':-15}
  </code></pre>