question: How many months after Engelbrekt summoned representatives from the four Estates to a Diet in Arboga was he assassinated?
#1 return Engelbrekt summoned representatives from  the  four Estates to a Diet in  Arboga 
#2 return Engelbrekt was  assassinated 
#3 return month of #1 
#4 return month of #2 
#5 return the  difference of #4 and  #3