Suppose there's a round pond, a frog which can swim as 1 meter / second, and a snake that moves along the pond ridge but cannot swim.
If the frog can reach any point on the ridge of the pond before the snake, it escaped successfully; however, if the snake is waiting at the ridge, the frog will be bitten to death.
Now, what is the maximum value of the snake's speed, that the frog still have an escape strategy?