If the circle x2+y2=a2 intersects the hyperbola xy = c2 in four points (xi,yi), foci = 1, 2, 3 and 4, then y1+y2+y3+y4 equals
If the circle x2+y2=a2 intersects the hyperbola xy=c2 in four points (xi,yi) for i=1,2,3, and 4 then y1+y2+y3+y4=