How many years after the population of Gdańsk was massacared was a momument errected to commemorate the event?
return the  population of Gdańsk was  massacared ;return a momument was  errected to commemorate the  event ;return year of #1 ;return year of #2 ;return the  difference of #4 and  #3
['select', 'select', 'project', 'project', 'arithmetic']