Alice and Bob are participating for this year’s HAKA-LAKA contest. Both of them qualified for the final with their talent. Now in the final, they got a very easy geometry problem to solve. Whoever can solve the problem will be this year’s HAKA-LAKA King. So the problem requires one to find the radius of a circle where $AB$ and $CD$ is perpendicular and the length of $OA$, $OB$, $OD$ are given. Bob doesn’t know how to solve this problem so this is the golden opportunity for Alice to win. Since you’re Alice’s friend, help Alice to solve the problem.

This is a companion discussion topic for the original entry at https://toph.co/p/haka-laka-king