IMO 1998 Problem 5

Let $I$ be the incenter of triangle $ABC$ . Let $K,L$ and $M$ be the points of tangency of the incircle of $ABC$ with $AB,BC$ and $CA$ , respectively. The line $t$ passes through $B$ and is parallel to $KL$ . The lines $MK$ and $ML$ intersect $t$ at the points $R$ and $S$ . Prove that $\angle RIS$ is acute.