sum(Sum,X,Y):- 		Sum is X+Y.

sum(Sum,X,Y,Z):- 	sum(Sum1,Y,Z),
			Sum is X+Sum1.

sum(Sum,X,Y,Z,Zz):-	sum(Sum1,X,Y), 	
			sum(Sum2,Z,Zz),
			Sum is Sum1+Sum2.