Single lane highway

 from itertools import permutations

import math 

# def get_count(d):

#     c=0

#     for i in d:

#         c+=1 

#     return c 

n=int(input())

l=list(map(int,input().split()))

cc=[]

# d1=permutations(l,n-1)

# d2=permutations(l,n)

# cc.append(get_count(d1))

# cc.append(get_count(d2))

s1=math.factorial(n)//math.factorial(n-(n))

s2=math.factorial(n)//math.factorial(n-(n-1))

cc.append(s1)

cc.append(s2)

if(n%2==0):

    t=sum(cc)+2

else:

    t=sum(cc)-1 

    

print("%.6f"%(t/cc[-1]))