Simple Billiard Table with Positive Lyapunov exponent

Question

I can understand the Lyapunov exponent for a circular billiard table is zero. However, if I were right, I read the Lyapunov exponent for a stadium billiard table is non-zero which was surprising for me. So I am asking is there any "simple" billiard table that its Lyapunov exponent is positive? By "simple" I mean one can prove it in an almost straightforward way like the circular billiard table that is just very easy.