Question 3
(UKR) IMO5. 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.