It's easy to understand.
Suppose the mouse runs on a circle with radius r.
The cat is chasing on a circle with radius r.
There's always a way to make an alliance with cats.
The distance between them is R+r instead of r-R.
At this time, the mouse is only R-r from the shore.
The speed of a cat should be 3. 14R/(R-r) times that of a mouse.
The result must not be 3. 14.