title: How to combine dataframes in specific situations?
body: <p>I have two dataframes as below.</p>
  
  <p><a href="https://i.stack.imgur.com/nKmfL.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/nKmfL.jpg" alt="enter image description here"></a></p>
  
  <p>I want to combine these two dataframes in the following rule.
  First, I want to focus on column A,B,C,and D.
  If those values are all same among two dataframes, I want to calculate the sum of the value. Also, for column E, I want to leave the df1 value.
  The expected outcome is below figure.</p>
  
  <p><a href="https://i.stack.imgur.com/h1y7T.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/h1y7T.jpg" alt="enter image description here"></a></p>
  
  <p>If you could help, I would appreciate it very much.
  Thank you very much for your help.</p>