A convex quadrilateral \(A B C D\) is inscribed in the circle with diameter \(A B\). Let \(A B\) and \(C D\) meet at \(I, A D\) and \(B C\) at \(J\), and \(A C\) and \(B D\) at \(K\), and let \(N\) be a point on \(A B\). Prove that \(I K \perp J N\) if and only if \(N\) is the midpoint of \(A B\).