question: Get sum of values of columns  'Y1961', 'Y1962', 'Y1963' after group by on columns "Country" and "Item_code" in dataframe `df`.
  
target: df . groupby ( [ 'Country' , 'Item_Code' ] ) [ [ 'Y1961' , 'Y1962' , 'Y1963' ] ] . sum ( ) 
  