import math

c = int(input())
rin, rout = [int(x) for x in input().split()]

r0 = (rout - rin) / 2
v = 0
div = (rout - rin) / 10**7
r02 = r0**2
rr0 = rin + r0
r = rin
for i in range(0, 10**7):
    r = i * div + rin
    x = rr0 - r
    v += r * math.sqrt(r02 - x * x)
v *= 2 * 2 * math.pi * div
res = v * c

print(res)