from decimal import *

D = Decimal
getcontext().prec = 100
a = n = D(1)
g, z, half = 1 / D(2).sqrt(), D(0.25), D(0.5)
for i in range(18):
    x = [(a + g) * half, (a * g).sqrt()]
    var = x[0] - a
    z -= var * var * n
    n += n
    a, g = x    
print(a * a / z)