There is a candy store near Mocha's school. It's said that the storekeeper, Dagashiya, can cast the railgun spell. To be the most powerful Mahou Shoujo, Mocha asks Dagashiya to teach her the railgun spell.
To test her mastery of this spell, Mocha creates a circular wall

whose center is
)
. The railgun spell has a
base segment 
with length

. When it is cast, a point

on the circular wall will be destroyed if and only if it meets the following conditions:
- The projection of point
on line
lies on the segment
(inclusive). -
make a counterclockwise turn, that is to say
.
Mocha chooses a point

which strictly lies in the circle and sets

as the center of the base segment. Then Mocha will choose an angle

arbitrarily and rotate the base segment around

by

. Because Mocha is a rookie of the spell, the endpoints of the base segment will always lie in

strictly, which means the distance from

to the wall

is greater than

.
Mocha wants to know the maximum length of the wall that can be destroyed in one cast.
You can see the note for better understanding.