How many years passed between the sending of the four peace missions to Joseon and the obtaining of the the promise of normalizing of relations?
return the  sending of   the     four peace missions to Joseon ;return the  obtaining of   the     promise of    normalizing of relations ;return when  was  #1 ;return when  was  #2 ;return difference of   #4 and  #3 in  years
['select', 'select', 'project', 'project', 'arithmetic']