Suppose the circle S : x² + y² + 2gx + 2 fy + c = 0 cuts orthogonally the two circles S' : x2 + y2 - 4x - 6y + 11 = 0 and S" : x2 + y2 - 10x - 4y + 21 = 0. If the centre of S = 0 lies on the bisector of the angle between the positive coordinate axes, then 2g + 2f + c =