R,O=range,print
T=10**12
D=[1]*14
A=D[:]
B=D[:]
for i in R(1,14):D[i],A[i],B[i]=D[i-1]*(3*i+1)*i*2,A[i-1]*(3-2*i),B[i-1]*i*2
N=int(input())
if N<1:O(0.4),exit()
if N<2:O(10/9),exit()
n=0
while pow(N,3*n-5)<=T:n+=1
n-=1
l,r=0,N
while l+1<r:
	m=(l+r)//2
	if m*m>N:r=m
	else:l=m
s,M=N*N*sum((D[n-1]//(3*i+1)//B[i])*A[i]*N**(3*(n-1-i))for i in R(n))*T,N**(3*n-2)
l,r=l*s//D[n-1]//M,r*s//max(1,D[n-1]-1)//max(1,M-1)
s=N*s**2
while l+1<r:
	m=(l+r)//2
	if (m*D[n-1]*M)**2>s:r=m
	else:l=m
a=str(l)
a="0"*(13-len(a))+a
a=a[:-12]+"."+a[-12:]
O(a[:-6])