how to add attribute value from two differenent examplesets

I have two examplesets with the same rows. They all have the attribute, for example "temperature". Now I want to add "temperature" attributes from two examplesets, then use the results to modify the "temperature" attribute in first exampleset. Two exampleset have the  ID attribute.

temperature in exampleset 1:            
20
30

temperature in exampleset 2:            
24
28

The result should be like:
temperature in exampleset 1:            
44
58

Could someone tell me which operator I should use?
Thank you very much