How many more million Mexicans live in the United States compared to Salvadorans residing in the United States?
return the  United  States  ;return Mexicans that live in  #1 ;return Salvadorans residing in  #1 ;return number of  #2 ;return number of  #3 ;return difference of #4 and  #5 ;return division of #6 and  a million
['select', 'project', 'project', 'aggregate', 'aggregate', 'arithmetic', 'project']