In a quadrilateral ABCD, the diagonals are perpendicular and intersect at the point S. Let K,L,M, and N be points symmetric to S with respect to the lines AB,BC,CD, and DA, respectively, BN intersects the circumcircle of the triangle SKN at point E, and BM intersects circumscribed the circle of the triangle SLM at the point F. Prove that the quadrilateral EFLK is cyclic . CyclicgeometrySymmetricUkraine Correspondence