title: Add 2 column from 2 dataframe in pandas
body: <p>I am new to pandas, could you help me with the case belove pls
  I have 2 DF:</p>
  
  <pre><code>df1 = pd.DataFrame({'A': ['name', 'color', 'city', 'animal'], 'number': ['1', '32', '22', '13']})
  
  df2 = pd.DataFrame({'A': ['name', 'color', 'city', 'animal'], 'number': ['12', '2', '42', '15']})
  
  df1
      A       number
  0   name    1
  1   color   32
  2   city    22
  3   animal  13
  DF1
      A       number
  0   name    12
  1   color   2
  2   city    42
  3   animal  15
  </code></pre>
  
  <p>I need to get the sum of the colum number e.g. </p>
  
  <pre><code>DF1
      A       number
  0   name    13
  1   color   34
  2   city    64
  3   animal  27
  </code></pre>
  
  <p>but if I do new = df1 + df2 i get a </p>
  
  <pre><code>NEW
      A             number
  0   namename        13
  1   colorcolor      34
  2   citycity        64
  3   animalanimal    27
  </code></pre>
  
  <p>I even tried with merge on="A" but nothing. 
  Can anyone enlight me pls
  Thank you</p>