title: Lookup from multiple columns python
body: <p>I have the following dataframe df:</p>
  <pre><code>       ID      03/01/2021     17/01/2021  20/02/2021  03/07/2022   17/07/2022  
  
    0   ABC             5               6          10           5            5            
    1   BCD             6               7           3           6           10              
    2   DEF            10               4           5           2            8              
  </code></pre>
  <p>I would like to look up for the columns that have the same month and year ,and save the sum of the values in a new dataframe.</p>
  <p>Output would be:</p>
  <pre><code>date       sum 
  01/2021     38     
  02/2021     18
  07/2022     36
  02/2023     19
  </code></pre>
  <p>I do appreciate your help!</p>